I think this is generally an option that needs to be added by the widget creator.
If you find the widget on your hard drive (Should be in Library>Widgets), however, and import it into Dashcode (you'll need to install the developer tools on your install disk, if you haven't already), then you may be able to resize it in there, but really, it's probably best not to go fiddling around with the insides of widgets (like it's best not to with applications). Make sure you make a back-up before you do anything.
Edit: I Just tried opening a widget in Dashcode. Seems you probably can do it. Just drag the bubble on the right hand side, I think... Dashcode is fairly easy to master, though. (The more point&click aspect, anyway)