I had a friend who didn't show until her 8th month. She had been a gymnst and because her abdominal muscles were strong she didn't show at all early on... But at 8 months she grew a big prego belly :)
I didnt start showing until about 7 months and I was worried , currently 38+4 and 165 pounds I was 125 :3
As long as you're healthy and baby is healthy and your care provider hasn't said anything you should be fine
Size wise? Some don't get very big at all.