My "one-cymbal" cymbal is a particularly magic 18" AA Medium Crash, which is a great ride when played with a lighter touch, and a great crash when laying into the edge. I think 18-19" is a good range for a crash ride, not too heavy for a crash, and not too light for a ride. I find that a larger crash may make a nice ride, but it can be overpowering (crash-wise) for the kind of gigs that benefit from a compact set-up. I have a 20" Vault Crash like that, but it only works well on slightly louder gigs.
Bermuda