These really work
I've put the viola stickers on a number of instruments. For most, the viola stickers fit fine but for about 20% of the violas I've had to put on a different size or split the sticker on the perforation. I usually have extra around so that I have that flexibility.
If you are order just one of these, I would definitely measure the string length on your viola and don't just look at the size. Don't Fret has string lengths listed for each size. On another note, I was pretty horrified when I saw these stickers the first time. It took me a few years but I decided to try it for my middle school program of 200+ students. I have found these tapes go on much faster than individual tapes and stay longer. Also, you don't get the buzzing from the individual tapes. There can be some problems with nuts that are too low, but otherwise it's super easy. The best part is communication with students. I can say a finger number, color and string and immediately get understanding. I can then go on to the more important things like finger patterns, key signatures, intonation, etc. Just an aside, I've never been one of those people who think finger tapes need to be exact because I've never been able to make them "exact". I tell the students the tape gets them close and they have to adjust using their ear. The Don't Fret tapes work great.