I personally think the ideal size of a siggy would have a max width of 500 pixels - this wouldn't stretch the forum page.
If you want to resize it, open it up in Photoshop (if you have one), and go to Image > Image Size, then put 500 pixels in the width box, and it would reduce the size without ruining the quality like it does in Paint. =]
Doesn't affect me much because I'm using a higher res on my screen, but when I used 1024 x 768 res, the stretching of the forum bugs me a bit.
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