If you take off the cover or grill (depending on what you have) you should able to measure to distance to the door card. Alot of people on here have upgraded their door speakers and as long as they are 13cm diameter then there shouldn't be a problem. I added speakers to mine as it didn't come with them originally, and I popped in a pair of pioneer ones, nothing special but they fitted fine.
Do a search on here for the grills, they are still available from VW.