I approximated 1 block width as a yard and the diagonal as 1.4 yards for ease of conversion. Home plate to backstop is 66' (about 16 diagonal blocks), to right field is 328' (109 blocks), to center is 400' (about 95 diagonal blocks), and to left is 335' (about 112 blocks). The fence is 9' high, or 3 blocks. The infield is to generic MLB specifications I found online, and everything else is approximated from pictures.
The map was created with the flatgen mod, but it is not necessary to install that mod to load the map. I don't know how to remove the flowers without mining them individually.
I'll add the rest of the stadium eventually in another map, probably with the home decor mod.

Here is the link, let me know if it doesn't work. This is my first time sharing a map. (edit 15 jul 14, removed flowers and adjusted portion of fence)
https://www.dropbox.com/s/m66hj9sjhlys9k3/baseball%20field%202014%2007%2015.zip
If you load in singleplayer, you may want to grant yourself flying privileges with the "/grant singleplayer all" command.