Originally Posted by MaysEffect

You do or don't have the speakers yet? The 6 1/2 is the largest circle size you can fit, 6 3/4 is the most popular size as you can fit them in incorrect size adapters and spacers. You can also fit a 6x9 or 6x8 component speaker, these are not that popular anymore, but JL and kicker made some of the best when they were.

5 1/4 and 1/2's are generally aimed towards larger systems that have dedicated subswoofers and mid range drivers as they have deeper cones per depth, resulting in slightly tighter, more accurate low bass and mids. The larger inch or so may throw out a bit more bass as a "do it all" speaker though, thus 6 1/2's are recommended for basic systems.