A screenplay is specific for anything on a screen: a movie or TV show.
A script can also include stage plays and radio plays to name a few.
The size of a screenplay depends on what your aiming for. Screenplays for feature films are typically 120 pages, but you can also write short films or whatever the size is for a TV program.
Take a peek at the tips thread stickied at the top of this forum, there are links to screenplay format examples, plus sites full of free-to-read screenplays.Just a question: when you say 100 pages, do you mean single-spaced, double-spaced, what size font etc.?