When do you know when it's time to up the amount of formula baby has at each bottle? We give her 5oz…

When do you know when it's time to up the amount of formula baby has at each bottle? We give her 5oz at the moment. She goes 2.5 hours between bottles (it used to be 4), but when she gets done eating she cries for a while, like she's wanting more. She still spits up, though, so I'm not sure if her belly would be too full on 6oz...