When to go up a size in diapers

My baby is currently wearing size 1 diapers, she's about 11-12 pounds and will be 3 months next week. What are some sure-fire signs it's time to move up a size? Babies-R-Us has a $10 off coupon for the huge box of diapers and I don't want to buy 260 size 1's if she's about to outgrow them. What do you all think? Could she possibly use all of those or should I just buy small packs of size 1's for now?Β FTM here πŸ™‹πŸΌπŸ™ˆ