Replacing rear speakers?

Hi all im currently almost finished putting my octavia back together and decided that as it was stripped down i would run all the cabling required to amp all the speakers in the near future (must say the easiest car to wire ever, its almost like they designed it to run chunky cables everywhere).

Read on here that the rears are 5.5 inch (i can see they are smaller than the 17cm fronts, but ain't measured them), but the golf and leon all have the same size front and rear as far as i can tell from a little searching.

Im now wondering if i should fit 13cm components in the rear or if i can fit 16.5cm ones and if anyone has changed theirs and can let me know what they did and what adapters they used (have everything for the fronts its just the rears i need to know)