I know that in my AE92 (4dr DX), the fronts are 4 inchers, and the rears were 5 1/2's. As for fitting 6 1/2's in the front, it'd be damn near impossible to do in the stock locations. If you were to do any size larger than OEM for the front, you'd have to dabble with fiberglassing some custom pods, maybe in the kick panel or the door panels.
Kenwood is an awesome brand, their execelon decks are top of the line, and offer amazing entry level decks for the price. I have a kenwood in my car, and I love it. But remember to not limit youself my name brand.
As for the Alpine speakers, they offer great products. The speakers differ in terms of quality and power handling, with type E being the entry level coaxials, and having their type x as the top of the line component system. It's all up to you what you buy, but if you are NOT using a 4 channel amp to power the speakers you purchase, I wouldn't go with anything higher than the Type R line (or any other brand of similar power handling). What you have to remember is that you are powering the speakers you buy with nothing more than the deck power, which in most cases isn't more than 20-30 watts RMS, if you're at a constant 14.4 volts. What this will result in is your speakers distorting after a certain volume level on the deck. This doesn't mean you can't power the speakers from the deck, you can under power speakers without any damage or issues, it's just that the higher up you turn your volume knob, the closer you get to distortion. Generally speaking, when powering speakers, you want to get as close to the RMS rating of the speaker as possible. What will normally happen is youll install everything and find a nice listening level, and eventually find the volume level on the deck that's about as loud as you'll ever really want to listen at. When you find the level that you wont ever really pass, a few clicks further and you'll normally run into distortion.
Another thing you should be careful of when buying is whether or not you're purchasing a COMPONENT set or a COAXIAL set. A COAXIAL set will be miles easier to deal with and install due to the tweeter being a part of the entire speaker. All you'll have to do with coaxials is buy the same size speakers as the OEM were, take the old ones out and drop the new ones in and presto. You're done. If you purchase a COMPONENT set, you may have to do a little custom fabrication. Component sets normally include a seperate crossover and tweeter not directly attached to the speaker itself. This means you would have to find a seperate mounting place for the tweeter AND crossover. Now, this doesn't mean you can't buy components, it just means theres a bit more work involved. I have 6.5 components in my rear deck that I made custom fiberglass pods for, and they're great.
Here's what most stereo guys look at when choosing components VS coaxials
Coaxials: Easy to install, get the job done.
Components: More work involved in installing, but a bit better sound quality due to higher grade materials going into the crossover and tweeter construction.
However, don't expect much low end coverage from the speakers you choose, for that you would need some stronger midbass drivers OR subwoofer/amp combo.
Sorry, I know it's a lot to read, but im sure it will help. Let me know if you need anymore help. And remember, DON'T limit yourself to namebrands! Hope this helps!
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