i think you're at the max it can be. they say your siggy can only be a max of 1/4 of the total screen. so if you divide the screen into 4 horizontally and it fits than you're good...
If you think your picture is too big, go to http://www.picresize.com/ and select your picture. In the quick options type 250px (max height) in the height text box. Click the resize pic and what you download will be the max allowed picture. If it comes bigger than what you had, use what you had.