It depends on the size of the field, and where you'll be shooting. At most venues I go to (colleges, schools), I can find myself rather close to the action. The best lens is an 80-200, possibly with a 1.4x on full-frame bodies. The second body has about a 400mm on it, for outfield shots, on the off chance there's action out there. And the ball travel time is long enough that swapping bodies off shoulder and into your hands can be done. In some places, I even keep a 12-24 mounted instead of the longer outfield lens, because I can get great crowd reaction and dugout shots.
I dislike images that are so close to the action that you lose perspective on what's going on, so I opt for a little wider view compared to too close. Most of the images will never see use larger than 8x10 anyway, intended for news use. I can crop as much as I want and quality is fine. If I know I'll be shooting for larger, I'm more careful about lengths. But still, I opt for a lens that lets me keep play perspective. A run-down, for example, loses all effect if you're stuck at 300mm and can only cover 1-2 of the players.
I'm curious how you can land a gig for a baseball league, having never shot baseball?
