They are standard dime store bandanas. I just measured a clean one, and its about 20" on each side (they vary slightly, but not much). And, like I say, after I've folded it on the diagonal, I roll the hypotenuse over once or twice (about an inch per fold), so that the loose part doesn't bunch up and make it look like a dinner napkin on a portly gentleman about to eat a lobster.
I try to keep the triangular part of the bandana on her back, boy scout style, rather than in front, coyboy style. It looks prettier that way, but of course gravity tends to make it to slip round to the front. Halfway round looks quite jaunty, but is for quick pictures only. On the other dogs I've known, position often depends on the dog's size and coat, and how long the bandana's been on the dog...fabric stretches on the diagonal, so they get loose with wearing.
If you want the ones you make to be truely reflective, skip the silver lame and get reflective tape from the sporting goods or auto sections of Walmart or other store of that type. It comes in day-glow orange or a glow-in-the dark luminous green, either of which would be really neat.
Actually, for serious visibility....hiking in hunting season....Abby has a safety vest. Its a long narrow triangle of orange plastic with luminous strips. The pointy end has a loop that goes around her collaer, and there is a velcro strap that secures the other two ends under her belly.