I resized mine to the size I wanted in a photo editing program, but if you print screen when viewing the image you would like (at the size you would like it - so zoom out and then print screen) and then paste it into a program like Paint, crop and edit it - then save it as your sig and upload that should do the trick hope that helps!
Yes you can do it on photobucket, click on your photo then go to advanced editor, and you can type in the size you want. Maximum 200 x 700 on this site IIRC
If you're struggling then post it full sized on here, and I can resize it for you