Re: 5.25 or 6.5 ??????
It is mostly due to space limitations, 6.5s will obviously put out more midranges, but will take up more room. Many times in kickpanels a 6.5 would take up too much room so the 5.25 fits beter. Then there are cars where the factory locations are only 5.25, But if you are asking about sound quality you won't tell much of a difference but 6.5s get a bit lower(depending on speaker) and will give ideally a tiny bit more output, but again its all theoretical, so get whatever fits the best in your car.
Alpine CDA-7995, A/D/S 3 Series Comps, ED 8A, SoundStream Reference 604, 98 Civic Sedan