Different forums can have different guidelines; I don't think we ever set any specific rules here at Oscraps. In general, your finished product shouldn't be wider than about 600px wide by 200-300px tall at 72 dpi. Mine is 500 x 167, for example. Otherwise they just take over your posts, in my opinion. I always start as if I were making a regular layout and then crop it down to the right size during the Save For Web feature (I use PSE). Some forums let you upload your siggy right from your computer but in this one, you need to upload the image to a hosting site like imgr or flickr; you could also use the gallery here as long as you aren't violating any gallery rules.
Does that tell you what you need to know? I'm not sure if I gave you enough information or too much, lol.