A full length movie of decent quality is around 700 - 800MBs. The problem your having is probably related to codecs.
Try using a different player.
VLC media player is a good all around player that works well for mac and pc. It also has the codecs built in.
Also check files with
Bitzi Web Lookup before downloading. It'll help you sort out if it's a good file or not. Also preview the file when it gets to 15% to check it's what the title says.
More info here:
CODECs list for Movies /Alternate players / Video help forums