A 5.25" speaker is a direct fit.
Most 6.5" speakers fit without modification other than maybe a couple new screw holes. However, some 6.5"s have a wider basket that would require either trimming the soundbar opening, or adapting a spacer ring to bring the speaker basket away from the hole.
If you are running the speakers off of the factory headunit, or an aftermarket h/u NOT connected to an external amp, a 5.25" is a good option vs a larger 6.5".
The power rating of the smaller speaker will be closer to that of the headunit. Most speakers of the same model line from one brand will be made the same (same construction, same material, same tweeter), so you're not really going to lose out on a great deal of attainable sound by not going with a larger speaker. The only loss will come from the smaller woofer moving less air. Coaxial seakers were never designed to be bass monsters, so I don't think of that as much of an issue.
If you are going to run an amp and no sub, I would opt for the larger 6.5". If you are going to run a sub (with speakers amped or not), I would go for the 5.25". With a sub, there is no need for the larger coaxial, and you can run the exact same speakers in the dash for a perfectly matched tone.