What you should do is cast on about 24 sts and measure the 10cm over the middle sts and don't include the side sts. They're a different size and curl under, so won't give you a reliable measurement. Then, wash and block the piece like you're going to for the jacket, and after it's dry, hang it up. Some yarns relax and grow with washing and with wearing, so you might actually be close enough with your size 4½s, or you may be able to use size 5s.
As far as making it larger, given the construction of it with the squares, it may be easier to use a larger needle, so you may be back to the 5½s.
sue- knitting heretic