First things first, format for screenplays is very rigid. It's way, way more than single or double spacing. There are multiple margins for different script elements, a font which must be used, and rules about everything.
If you're going to consider screenwriting, take a half hour and learn the rules. There are lots of places you can get them. You can produce a perfectly-formatted script in word processing, or use free formatting software Celtx, or pay for Final Draft or Movie Magic. I'd recommend beginners use Celtx, until you know if you're going to stick it out long enough to be worth the investment in software.
Once you've got the format down, yes, a short script (comedy, horror) tends to be right about 90 pages. You have a bit more room in a drama, up to 100 or so for an unknown writer.
Maryn, parroting the real screenwriters