If I recall the fronts are 4" but I have heard it said that 5.25" can be made to fit ( though I've heard a lot of things tbf lol).
As I understand it the sound reproduction is made up of 3 elements (treble, mid range and base) and a full range speaker (3way) will do pretty much the full range, where as the 2 way speakers only do the mid range and treble, so to fit 2 ways you would be lacking any base in the front, and also you would need a crossover to remove any base from the feed to the speaker, so as not to damage it. I think some may have a very simple crossover built in, but certainly most don't. Similar goes for a component, although some do come with tweeters, but again. That leaves you lacking base.
EDIT: Just seen Oranoco's post above regarding the 5.25's
This is only what I understand, and wait to be corrected lol