The following has officially been added to the posting guidelines:
Signature Size Guidelines
You may use one signature graphic no larger then 600 x 100 pixels and a line of text along with the image. You can have up to four lines of text if you prefer text over an image. Please do not use extra large text.
To check the pixels of your signature, right click on it. You'll see "properties" near the bottom of the popup window. Click "properties" and you will see size and dimension. Dimension tells you how many pixels your signature is.
Thanks everyone for all the suggestions on size