That is too broad of a question to answer correctly without guessing.
Do you want to stretch that element so that the nav tabs stretch the full length or do you want to stretch the container to full width while leaving the width of the actual tabs wrapping elements the same?
Are you using/modifying the default style, or a custom one?
These are things that will help provide you a real answer that is actually relevant to your needs instead of a shot in the dark that may or may not actually be what you need.
Do you want to stretch that element so that the nav tabs stretch the full length or do you want to stretch the container to full width while leaving the width of the actual tabs wrapping elements the same?