Freaking small is right. You're better off putting the 4 alts along the top or bottom, or splitting the screen in half (left and right), and putting the main and healer in the left, and 3 dps in the right.

1680x1050.. something like:

main on bottom: 1680x630
alts on top: 420x420 each.

or..

left:
main: 1120x600 (bottom)
healer: 1120x450 (top)

right:
dps: 560x350 each. (top, middle, bottom)

With a few add-ons, and some work to customize the layout, the 2nd option could work very well given the lack of space.
- Unit frames to clean up the default interface frames. (X-Perl)
- Action bars to clean up the default interface bars. (Trinity)
- Compact the bag space needed (Bagnon)
- Hide the chat/combat logs, xp bar, remove the default character bar (character, spellbook, talents, etc icons).
- TitanPanel to display info: xp, durability, gold, bag space, etc.
- Clean up map (basic minimap square & mapbuttonbag).

My current laptop is 1680x1050, but I'm not going to bother messing with this.. 2GB of RAM and shared-memory Intel graphics tells me I shouldn't even bother trying to load 5 instances of WoW, much less attempt to play. I've been considering what I'm going to do when the holidays roll around, since I'll probably take a full 3 weeks off from work. I may have to break down and get a desktop replacement laptop with 1920x1200 resolution and create a custom UI strictly for 5-boxing on that small of space..