You probably need a wrapper div around the content, set to a width that matches your design and that will work in most viewport sizes. If you are designing for the viewport being at full screen it is very common to run into this sort of problem. But as you are at least testing it by reducing the viewport size, you are thinking of other users, which is a good start.
drjohn 56 Posting Pro in Training
diafol commented: He asked for it. Ha ha ha. +6