I'm about to make a sig and I was wondering the size limits..

There really is no set limit. Keep it reasonable. Have a look at everyone else's images to get an idea on what is considered average and go from there.

I can also resize and optimize it for you if you wish.

How's this I made it around 400x150. Not my best work but quick and fun.

Nice job. Looks great.

I made this for a clan I'm in I think it's much better.

Personally I like the blue one. When I get time I will have to mess around in Photoshop and make me a cool looking sig one of these days....when I find time.