Hi

You can get a 5 1/4 inch (13cm) speaker in the front if you already
have the pockets. There is a huge choice of 13 cm speakers so it's
really up to your budget, go for a coaxial not a dual cone. Adding a
component set to the front will really improve and lift the sound but
also cost more and mean you have to install the tweeter somewhere.

The rear positions will take a 6X4 inch speaker in the standard
position. Quite limited on choice here, I think Pioneer still make the
TS-4628 speaker. Or you could put 6X9 cut into the rear shelf which
will give lots of volume and are great if you are on a budget. The
rear shelf on the golf is full of holes however and could always do
with a bit of MDF adding to it if you are going to cut speaker into
it. (again all more install time and money)