Funny thing about that....Back in the "old" days, the use of a cymbal wasn't specified. In essence, they ALL were crash/rides! There weren't a lot of larger diameter cymbals, either. Most were on the small side and used for quick embellishments. Over time, the music changed and so did the instruments. Eventually, cymbal makers started to expand their lines and models to the point it's at today. Along the way, models got named...uses became refined and categorized.
So, the end result is that we now have cymbals with more of a specified nature to them even though they all are still crash/rides. Actually the terms have to do with HOW a cymbal is played -not what kind of cymbal it is.
I buy cymbals that can be used for either purpose. I usually settle on 18" diameter and larger and try to find the lightest vintage Zildjians I can get used. Roy Haynes made the Paiste 602 line famous with his 18" flat ride.
Generally speaking, today, anything under 18" is probably considered a crash and anything over an 18" is likely a ride....An 18" diameter seems to define that "crash/ride" territory...but it's all just terms.Burger Kin