the reason it's defined as a number of pixels was some crap ie7 issue and the onlyway to fix it for the small ammount of users who where having the issue was to patch it and tick the rest of us off ;)...
You can change this to the original manner by changing the "ubbcode-body" class to:
margin: 4px;
max-width: 100%;
max-height: 300px;
overflow: auto;
font-size: 10pt;
display: block;
It looks MUCH better, and handles sub-items much better as well (sub-spoilers, sub-quotes, etc).
IT looks extremely odd when quoting a quote with the "new" set, but the "old" one it looks quite clean.
The way it looks now is:
The way it used to work (how the css i provided above) they would all "flow" together, and look crisp and clean as can be.