Bridal shower: There may be more than one shower, which makes it easy because people fit nicely into categories: work, church, bookclubs, sororities, etc. In this case it has a lot to do with your hostess...she will decide on the theme of the shower which may affect the amount of guests.
Your Kid's Party: Of course, invite by age...that is the easiest way. This is hard, because you may want to invite YOUR friends' kids rather than your kids' friends. I think thats fine when they are little...(ok thats all I know) The biggest thing is invite as many as you can handle and have enough adults there to help!
All other get-togethers: (Dinner Parties, Girls Nights Out, Date nights, etc.) I may be crazy, but I still feel like more is better. If it means being a little tighter around the table, its a small price to pay to enjoy more friends!