Hi, we have a Maximum Signature size of 200 pixels high by approx 1500 pixels wide.
If your Sig image is bigger than this then yes you may suffer some clipping off the bottom of the screen.
The reason the limit was set was because members were using very large images.
This was taking up server resources and making the reading of threads difficult.
for example someone posts a jokey picture to which most people would reply to with a smiley.
But for every smiley posted you then had to scroll down 1000 pixels of sig image to get to the next comment. . . tiresome.
If you need a more technical answer I've Tagged Marc and Jos our Tech Wizards
Hope this helps