Please note the following-
640 x 480 is the preferred and optimum size and also the one that larger pics are re-sized too automatically on here..
So, to sum up
Any pictures posted using photobucket, will automatically be resized to 640 x 480, regardless of their size unless its smaller than these dimensions.. I've just looked and photobucket has, yet again, changed slightly from when I did the guide... to alter the size of your pic on photobucket, if you are presented with a screen that looks like this-
click on the more options link (arrowed), which will then show this menu of available size options-