Forum signatures are allowed to be 300 pixels tall by 500 pixels wide. This includes all text on the signature.
300x500 looks like this:
You can find out what size your image is either by looking at the properties in Paint, the image size in Photoshop and Gimp, opening it in a new tab in your browser (it'll be on the tab), right clicking and viewing the properties in your browser, or looking at the properties in Photobucket.
Fonts sizes are as follows:
Thereby, two lines of standard forum sized text (Size 3 from the drop down menu) would be 30 pixels tall.
Signatures are NOT allowed to be animated. Glitter text DOES count as animation.