In this video I'll be showing you how to attach new wires into a harness, and splice in new terminations to wire up your speakers. I'm doing this on my 1999 Suzuki Jimny but the process will be the same for just about any vehicle.
The sizes of speakers that would fit in any particular vehicle definitely depends on the vehicle in question. In the case of my Jimny, I don't think a 6 inch speaker would fit. You'd have to cut the metal mounting hole larger, and I think it would be unlikely that the cover would still fit afterward. I'm not an expert when it comes to mounting larger than factory speakers, so I can't offer you very much advice on that. Sorry, friend.