My DD had a sleepover with 5 girls for her 8th birthday and I thought it was just the right amount. No one got left out and all the girls got along. A couple of suggestions:
Gave the girls a digital camera and then printed the pictures at home. They made little scrap books to remember the night. Got little scrap book kits off of the clearance rack at AC Moore.
Got a jewelery making kit from Oriental Trading - girls made their own bracelets.
Got canvas bags from Michael's and had the kids pick an iron on to put on it and then they decorated with fabric markers. They were able to put everything in the bag to take home. Just small bags, nothing huge.
Kids loved the crafts and my daughter still flips through the scrap book.
Good luck and have fun! They grow up so quickly!!!