Thanks ladies! Sorry couldn't see it for looking and I was sure Louise would know! Oscar is probably over 7lb but they leave marks on his legs and I'm not sure they fit! It's been a long time since I've used fitteds but I'm sure they look too small.
I've got smalls but they are too big, I wonder if I took photos if someone could tell me?!
Thanks @Hannahboo, I will remember that for next time