HOW TO CHANGE THE SIZE OF YOUR SIGGY WITHOUT HAVING TO RESIZE AND REPASTE ORIGINAL This is only easier IF you are HTML savvy already
HTML doesn't understand the height property of an image link when used in HTML. You you must use the width property (width="x") to control the height of your image proportionally. Remember that if you have text above or below, the size requirements INCLUDE the text, so you have to shrink you images down even smaller.
Steps to reduce your signature to a maximum of 100 pixels in height:
1. Determine the size of your current signature photo(s).
Example: My image is 96 pixels wide by 160 pixels in height
2 Divide 100 by the your image height. Your Image Height / 100 = Percentage
Example: 160/100 = .62
3. You must determine the new width of your image to proportionally shrink your images height. Image Width x Percentage = New Image Width
Example: 96 x .62 = 60 (New width of your image)
4. Insert the new width of 60 pixels into your image links like this:

typo: You can play with that number to GET the height down.
If you have more than one image, you must do this for each of your image links.
OR
You can wing it a bit if you have the Ruul Screen ruler for Chrome and do what I do. I measure the the width and height, and then adjust the width down until the height measures at 100px with Screen Ruler tool. It usually takes me a couple of tries.