Adjusting filesizes of highly compressable movies

I'm sure that more that a few people here have problems with undersized files using the 5.02Pro codec. I encoded Enemy atthe Gates over the weekend and was consistently getting undersized files. Hate that! I tried various combinations of resolution, bitrate, audio sizes and finally, after four encodes, got my 700MB. Totally frustrating! What I'd like to know is.... How does resolution affect file size? Obviously bigger resolutions elicit bigger file sizes, but is there a way to mathematically figure out the final file size based on a given bitrate and resolution? Usually I just fiddle with bitrate if the file size is an issue, but with the 5.02Pro codec sometimes it doesn't need as many bits as you give it to encode at a certain resolution. Thus smaller than desired file sizes. I've read the sticky notes and used search to find answers, but still am unclear. Is there an easy formula to follow?

Just a small correction: resolution DOES have influence on size, IF you encode in quality based mode. The bigger the resolution, the bigger the bitrate, the bigger the size. When you get undersized files, it's like you were encoding in quality based 100% mode: "perfect" quality! ;) [EDIT] You may define it that way: when you reach the desired size, the quality isn't optimal. When you get an undersized file, your rip is perfect!

@Acaila - thats what I thought - of course if you was trying to get the "best" possible encode quality without worrying about filesize - you'd use the highest resolution without going above the original size (740x). correct? Mac