I dont think the wall dimensions have any effect on the sim. You can set the wall as short as you want, to the point every hit would be a home run. Doesn't seem to change the game for me.
Now the batting average, doubles/triples, HR average definitely affects gameplay. I set my park to be as hitter friendly as possible and it shows. 30th in ERA, 1st in AVG, HR, Runs.
My best game to date.