There is no "correct" size. Assuming your glazing units are 4/16/4 (a
common size) plus a further 20mm for the glazing bead, that's a 44mm
deep rebate, so you might use something like 75*50mm section for your
opener. Frame depends on whether you go for a flush opener, or some
other pattern. It may make sense to match it to the original for
easier installation and making good.