I make bandanas for my girls myself. I only need about 1/4 to 1/2 yard for 2 bandanas. My two aren't that big so it all depends on the size of the dog's neck. You can measure around the neck to get a approximate size in inches that you would need for the length. I then just traced out a pattern, a triangle shape and cut it. I had to try it a few times to get it just right, but now Sadie & Maggie have about 20 each, all matching. It was all just trial and error for me until I was satisfied with the sizes. When I make new ones I just grab an old one and trace off of the old one and then I have a new bandana without the hassle of sizing again. I don't know if this helped at all, PM and maybe I can help some more. Here are some pics of the girls with their bandanas.
Sadie & Maggie