I would get a large and a medium first. And the sewn ins are a better fit.

I say that because Ash is 11kg at 10 months, and he's been in larges for... well since about 5 months.

Try pick up 2 second hand to try before you spend big $.

Also be warned, ittis don't like driers (half mine are crinkly and delaminated, the other half are fine), and they stain like a biarch when you use panadol.