For some reason, the portion of the NCAA site containing the rule book isn't working this morning. However, this article suggests that D1 baseball tournament rosters are capped at 27. The entire "travel party" is capped at 35. I assume this includes coaches, trainers, and non-roster players. Of course, 35 is just the number for which the NCAA will reimburse expenses. The school could always carry more than that, at its own expense.
NOTE: Take a look at the Travel Party sizes for football and basketball. Easy to tell where the money is.