Its DEFINITELY possible to limit image sizes. I know that on the Black Mesa Forums, there was an issue with sig-image size before I was banned for NO REASON. Raminator, one of the main mods, just made it so that every sig image on the forum over a certain size was resized automatically. I personally just used AdBlock Plus to block large sig images.
I don't exactly know about forum code but I don't think that it would be too big a pain in the ass to just ask them how they did it.