Oh I love baseball!
Well it all depends on figuring out what you are going to build.
For the field only, 300 feet down the foul lines would work well in N Scale (1/160 for non- railroad modelers).
A huge advantage there would be that as you know there's a world of stuff readily available.
And your dio would be around 2 feet square which is plenty big.
The next choice would be 1/192, or 1/16" = 1'-0". There you'll get 40 scale feet all around the field on the same base. Enough for some bleachers and a press box.
At that scale you can find a fair amount if stuff from architectural or ship model suppliers.
A full sized ballpark like AT&T which I helped design is over 1000 feet on a side. On the same base that's 1/500 scale or smaller. Doable if complicated and time consuming, but then a curiosity comes up- the diamond will only be 1 3/4" on a side, unsuitable for your purposes.
I'd love to see this come together!
If you go the other way with scale, remember it's the law of squares. A 3' x 3' base is 9 square feet and 125% bigger than 2' x 2' therefore that much more work. But I suppose most of it is grass so it's just more plywood and bags of grass.