Recommended easy fit is 5.25.
Up to 6.25's should fit no problem, some 6.5's should fit. For 4 doors, this is no big deal. For sports, the 6.5s don't fit to well unless you really cram them in there for the rear. This is due to that little grill, because after add in the adapter/spacer, it adds more height, to the speaker. Furthermore, I had to cut the inside of the grill to make it fit. It's totally unnoticable, and easy, but the 6.5 is so tight, that you can't get the grill back in under the panel. So I had to cut out the area on the griell that the speaker blocks. The largest size you can purchase, the better.
Your tweeters will depend, but flush mounts take up a lot more space than you might realize. For example, if you have a 1" tweeter, assume that you'll need at least 1.75" of flat space to mount it (1" for the tweeter + 0.75". Furthermore, you often must have at least 1" to 2" of empty space to mount it. If you just surface mount them, the decisions of positioning are much easier.