Eric Hosmer has long had one of the sweetest-looking left-handed swings in the Majors, and now that he's a free agent coming off a career year, it's tempting to think about what his power numbers might look like in another ballpark, rather than in spacious Kauffman Stadium, which boasts 118,500 square feet of space in fair territory, the most in the American League.
It's not easy to hit a ball out of Kauffman, which is why Steve Balboni's 36 home runs in 1985 stood as the Royals' record for more than three decades until Mike Moustakas bested it in 2017. While the average fly ball/line drive in Kansas City was hit 286 feet, almost exactly the Major League average, the average home run there was hit 409 feet, tied for the second-longest in the Majors behind the mile-high altitude in Denver. If you hit a homer in Kansas City, you earned it. You had to.
So it's fair to ask whether Hosmer would hit more home runs than the 25 he had in both 2016 and '17 if he were in a more hitter-friendly home park. Just as we did with Giancarlo Stanton prior to his trade to the Yankees, we can use Statcast™ to try and find out.

Hosmer is something of a unique case, because despite his slugging reputation, he had one of the highest ground-ball rates in the game, and grounders don't become homers in any park. When he did put the ball in the air, he rarely pulled it; just 13 percent of his air balls were pulled, compared to 51 percent that went to the opposite field. Hosmer was very successful the opposite way (he hit .522 with a .973 slugging in the air to the opposite field, third only to J.D. Martinez and Aaron Judge), but it might change the way you think about how ballparks affect him.
We've taken the 41 batted balls of at least 300 feet of projected distance (potential home runs) that Hosmer hit at home in 2017 and mapped them on 3D rendering of the parks of three teams that are often named in rumors: Boston, San Diego and St. Louis. Note that we're just looking at the actual trajectories, not accounting for potential changes in temperature, elevation, etc., so this is more of a step up from the old 2D images we're used to rather than a true projection system.
Fenway Park, Boston

Fenway might give Hosmer all of one more home run, and take away perhaps two, though even those two are within the "likely" zone of uncertainty. You're stunned. We can explain.
Since Hosmer does have that nice lefty swing, you can just imagine him taking aim at the right-field Pesky Pole, just 312 feet away from home plate. But this is where Hosmer's batted-ball tendencies really come into play, because he rarely hits the ball in the air to right. Of the ones he did hit to right, they were all hit so hard that they'd be out of both parks, or not hard enough to be out of either park. (The one exception there is his Sept. 8 home run against the Twins, a projected 376-foot blast that may not have cleared the 380-foot fence in Fenway.)
We're actually more interested in the Green Monster for Hosmer, since his air balls go to left, and it seems like he might have lost one homer to the Monster and gained one back to center. The one he might have lost, hit in late August against Dan Jennings, potentially clips the top of the Monster in left-center if hit in Boston.

That's not much of a change. But remember, it's not just about homers. It's about extra-base hits, too. Would Hosmer have turned outs into wall-ball doubles?
That's a little trickier because we aren't accounting for the defense, but here's what we can do. Of those 41 balls of at least 300 feet, 19 were outs. Ten of those 19 went to the left fielder, and of those, three didn't travel the minimum 310 feet to get to the shortest point of the Monster, at the foul pole. Three others went between 310-320, but enough toward center field that they likely wouldn't have hit the wall.
So we're left with potentially four outs that might have been doubles in Boston, like this September liner that went a projected 354 feet to David Peralta.

Four doubles isn't nothing; it'd turn Hosmer's .318/.385/.498 slash line into .325/.390/.511, which looks nice. But because we're not accounting for the defenders, we're not guaranteeing all four would be doubles, anyway. Unless he changed his approach to minimize some ground balls, Hosmer in Boston might look similar to Hosmer in Kansas City.
Petco Park, San Diego

Despite the fact that the Padres have a first baseman in William Myers and seem to be in the midst of a rebuild, reports indicate that they're seriously interested in adding Hosmer, and the shorter dimensions in San Diego might be a boon if he went there. Based on what he did in 2017, Hosmer might add two to four home runs and not lose any.
For example, on May 3, Hosmer crushed a Jennings pitch (yes, the same Jennings -- he was traded from Chicago to Tampa Bay in July) at 106.8 mph and 24 degrees, sending it a projected 410 feet, with a Hit Probability of 96 percent. Unfortunately for Hosmer, White Sox center fielder Leury Garcia made a great play on it, turning it into a sac fly. It wouldn't have been a homer in Kansas City anyway; it might have been in San Diego, and is the green line you see above for Fenway, too.

Again, we'll note that in the early stages of this tool, we're not accounting for environmental changes, and the air in San Diego isn't the same as the air in Kansas City. That said, Petco Park is no longer the extreme pitcher's park it once was and still has a reputation for being. The fences were brought in prior to 2013, and the '15 additions of a new video board and a large building beyond center field have changed the wind patterns.
That being the case, a ball like this one, which was "only" a double in June when Hosmer crushed it a projected 393 feet off of David Paulino, might have instead cleared the fences in San Diego.

Busch Stadium, St. Louis

Though the Cardinals have already added Marcell Ozuna to reinforce their lineup, they don't appear to be done just yet, as they're reportedly still talking about third basemen Manny Machado and Evan Longoria. But they have another option, too; they could sign Hosmer, push Matt Carpenter back across the diamond to third and make Jedd Gyorko the flexible infield backup he's best suited to be.
While Busch is six feet deeper than Kauffman at the left-field foul pole, it's generally five to 10 feet shorter all the way around, with equal eight-foot wall heights. That means that Hosmer probably wouldn't see much change here, either staying equal or gaining potentially two homers.
This ball, from June 2 against Cleveland, was hit hard (103.1 mph) and far (projected 387 feet). In reality, it hit the Kansas City wall and bounced for a double; in St. Louis, it would possibly clear the shallower wall.

Let's remember that the Hosmer we saw in 2017 was generally a very good player, as his park-adjusted batting line made him similar to Daniel Murphy, Anthony Rizzo and Edwin Encarnacion. He was even better for most of the year, because after a dreadful April (.225/.281/.292), he hit .335/.402/.533 over the remaining five months, basically what Justin Turner did in 2017.
But barring a significant change in the way Hosmer approaches the ball, no park will do much to affect his power numbers. There were 110 players who hit more than the 41 home batted balls of 300 feet that Hosmer had, including Royals teammates Whit Merrifield (71) and Alcides Escobar (58). If the ball isn't in the air, then the dimensions of the field don't matter all that much.