I tend to do mine as 1600 x 300, but I don't believe there is a set size that you have to use.
You can also use the fastdl siggen signatures to have signatures with stats from ingame, which can be found by using the following link:
http://fastdl.friendlyplayers.com/siggen/<servername>/<steamid64>.png
<servername> should be replaced with one of the following: darkrpbase, ttt, prophunt, murder, surf, deathrun
<steamid64> should be replaced with your SteamID64.