How can you tell which bottle nipple your baby needs? ?

My 2 month old has had problems with crying during feedings. The pediatrician decided to put her on a new formula, soy based. She just tried it today and she is still crying during feedings. It could be the nipple. I was hoping one of you knew which size are appropriate for a two month old. We are using the even flo and playtex. Should I be using the slow flo or medium flo do you think? I have to figure something out. She did the HUGE burp that let me know air is in there, either from gas or the bottles possibly, I just need some suggestions.