on photobucket above your picture it should say "edit". then goes into another screen and above your picture there should be options like resize / rotate etc.
obviously then you just pick the resize one. in the top right hand corner of the picture, if you hover your mouse over the picture, will have a percentage.. and that shows how big the image will appear to other people when you use the link. so just resize it down until it says 100% on that and it's the best size for you.