To check size:
Right click any image in your siggy,
Scroll down to Properties and click.
You will get a message giving the size. 319px × 164px (In this case Width is first, Height is 2nd)
Remember, for two or more images that are stacked, you must add their heights to get the total height.
Likewise, for two or more images that are end to end, you must add their widths to get the total width.