What needles

[IsThisAkissingBook]

Will I need to knit a dog jumper in the round? I've got regular circular, is it 25cm fixed ones that I need?

Think you need to tell a bit more. Depends on size of dog, ply of wool. Are you following a pattern? There are lots free on Ravelry.
I knit lots of jumpers for my dogs but, personally, not keen on knitting in the round so usually knit the back & tummy piece as two separate pieces then join them leaving a gap for the front legs.