Brake and fuel tubing is sold as OD size. The 1/4, 3/16, are OD sizes. The handheld straightener you are considering is sold by OD as well. So if you are buying bulk 1/4 and 3/16 stainless brake lines, you will need one of each of the straighteners/benders. If you are putting new 3/8 fuel supply line in, you will also need a 3/8 straightener/bender.
Or.....in case you didn't know, you can also buy pre-bent sets if that makes your life any easier...