yes you can decrease the filesize but at the cost of slightly to noticeable quality loss depending on how much you compress it (or reduce its bitrate) and lowering the resolution is a good idea when reducing bitrate
if your motivated to learn try Handbrake here is a guide i made for mini-size encoding of fansubs -
https://sites.google.com/site/taumox/handbrake-tutorial
or if you want more simplier method try my custom GUI for Handbrkae Cyko -
https://sites.google.com/site/taumox/cyko-tutorial
P.S. im no expert btw im just a regular reader of basic video encoding (doom9, doom10, animesuki fansub section)