Pretty much every place I have shopped for bags has listed dimensions (REI, NRS, etc.). If you see a bag and don't see the dimension find the same bag on another site for dimensions. Either REI or the manufactuerer will generally have the dimensions IME. Keep in mind that, assuming a roll top bag, within reason you can just roll the top down an extra roll or two to get to the length you want. Just don't go way longer than you need.