They can be edited if I'm not mistaken. But I don't have nearly the experience to do this. Basically, what needs to happen is you need to access the wall.dat in the stadium's .big file. Once you export that as a Notepad document, there's a whole bunch of numbers that correspond to the coordinates of the wall layout. It's how the game detects whether a ball is fair or foul, if the ball is in play or out of play, it's a homerun or not, etc. That's the jackpot for getting the proper dimensions, but it's just a slew of numbers without explanations, so good luck.
Then, if you're able to