8p 3DR Speaker size

Fronts are 6.5" components with 1" tweeter
Rears are 6.5" coaxials
You'll need adapters to fit both fronts and rears
Wattage is irrelevant as they don't make a speaker so puny that it would be overpowered by the 4 x 20w output from the HU & rear amp
Be aware that if you have removed the stock sub then you have also removed the rear speaker amp (inside the sub), so if you are keeping the standard HU then you will have no feed to the rear speakers. You will need to either extract the amp from the old sub and reinstall it or add a small aftermarket amp to power the rears. Or better still forget about the rears altogether and spend more of your budget on front components, or even better still a decent amp to drive them.

Should really have got the MDF ones. You need proper 2 way components, pretty much anything will fit

Yes, as said you need proper 2 way components and from a decent manufacturer, if you're on a budget then something like these Alpines:
Alpine SXE-1750S