While still in Photobucket look up where you see the word "Upload Images and Video'?
Right below that is the words "more options" in green.
Click on that.
It will give you options in the size that your photos will post.
You want to make sure you chose "Large 640 x 480" as your image size.
This will give you a nice large photo without being too big where you have to scroll back and forth to view it.
You only have to do this once and all your photos will stay this size unless you go back in and change it.