Cool guys thanks for the tips.
Size wise I think I'm doing alright, already got the thing down smaller than any others I've seen down at 100x308. I'm definitely using the save for web feature, problem is even though the sig is quite small I had to save it on a really low quality down around 30ish, you can even tell looking at the sig that it's fuzzy, it looks much better when I save it with higher quality.
I'm sure you're right about the explosion taking up all the space, but don't think there's a lot I can do about that. It's really the basis of the idea I had for the sig, playing off the nickname Fason has of C4 and putting in Kaboom or Boom or something with an image of him running someone over. Oh well I'll just have to go with the low quality I guess. I had flattened the layers and all that as well.