You can even export it as "uncompressed" if you like (it might be several GB in size, but that's okay). Then open it in MPEG Streamclip (
http://www.squared5.com ) or use Quicktime Pro. Make sure you save it in the right aspect ratio. I usually export it out of iMovie or Final Cut using "custom" dimensions, and then I manually type in the pixel width and height, making sure it's correct for the aspect ratio. I cannot
stand incorrect aspect ratio!
Some settings that I use with good results are H.264 for compression ( or the x264 codec works faster and is free). The XviD codec does pretty well for me. Also, you can pay for the Divx encoder.
This guy (a member of this forum) has written a fantastic Guide for Macs. Here's the page about DivX.
http://www.ishtori.net/index.php?id=27
I use a bitrate of anywhere between 900 to 2900. With MPEG Streamclip, you can type in your bitrate limit and it'll give you an approx. file size. I usually encode in the 1500-2300 range.
If you want to make things easier for yourself, fork over $23 for
VisualHub. It takes sort of a "Dummies" approach to video compression, and I find that it sometimes softens the video—and I'm not crazy about its WMV files (but who wants to use them anyway). But if you feel like tweaking with it, you might really like it. Works fast too.
*Ahem* Sorry, more than you really asked for (or probably needed) eh?

I get carried away.
In summary, check out the Mac guide I linked to. It's fabulous.