I don't think I've ever heard an 18" crash that wouldn't work in that role (thick/thin, b8/b15/b20, whatever). I prefer larger cymbals (18" +) but have crashed away on cymbals as small as 15" and the point still gets across (though not quite as loudly). It's not all that different than bashing away on open hi hats if you think about it.
One the thick vs thin consideration: thinner cymbals work well but you have to play straighter 8ths volume-wise to keep the dynamic up since they decay so quickly. Thicker cymbals work just as well and give you more room to relax since their inertia will help keep them loud between quarter notes.
Beyond that, only thing you really need to care about is that cymbal sounds good - to you.