To expand a little, you should be able to right click on the image in your sig and select properties, if you're using Internet Explorer.

If the number in bytes is over 50000, then it's too big. If the dimension says over 550x250 then the size is too big. I don't know how you check with other explorers. If you have the image saved on your disk, you can check by just selecting it in the folder it's in, and checking it's properties there.