The reason you want your minor leaguers to have roster sizes of at least 35 is because there is no minor league DL in
OOTP. So, if 2 of your AAA pitchers are injured, they stay on the roster ... and the AI will not demote them (or anyone else usually) to bring up 2 additional pitchers to help out.
So, you need to have spaces available for extra guys to cover this -- else you'll find your remaining pitchers getting seriously overworked. I say keep your minors set to 30, though, and monitor injuries. If it appears that injuries are an issue, raise the roster size by 1 or 2. 45 should be good at the lowest level, but you may find that AI-controlled teams sign hordes of a minor leaguers and releasing hordes of minor leaguers all at once.
The AI is pretty good about balancing rosters at all levels other than the lowest. Marcus has a post here somewhere where he explained why that is (including the large signings and releasings), but I can't recall the answer specifically. Basically, though, you'll see pretty good roster managment at all levels other than the bottom, where you definitely need to keep roster limits high (unlimited is suggested).
As for salaries, the numbers you enter are averages, not caps. So, a $14MM superstar average means that some superstars will make more. How precisely the game determines the salary curve I'm not sure.