If you want to know how big the signature is, (in most modern browsers), right click on the image and click Properties. It will then tell you how big the image is. The same steps can be used if you're looking at an image on your computer and not the Internet. Programs like GIMP and Photoshop make you decide how big the picture will be from the very beginning in the event that you're making a signature.
If you have any more questions, feel free to ask.