Taran
My Yellow tang looked like that when I purchased him from the pet store. I took him out of a small 20 gallon tank against my better judgment, but I couldn't stand to see the guy like that. I only have a 55 gallon tank and he is about 5 inches long now and all his fins are grown back.
He is healthy and looks like he is having a good time swimming between the rocks. So does size matter? I would say yes, but 55 is 100% better than the 20 gallon they had the poor fish in. He was put in there because the other fish attacked the guy. So he was in a 20 gallon tank with a Yellow Eye Tang. I bought both!
I am upgrading to a 180 gallon, but not for another year. What I did to get my guy back to health is Garlic Guard and Mysis Shrimp mixed with a good pellet. Plus Sea Weed on a clip every other day. I don't know if the Garlic Guard helped, but I am sure the diet did.
Also I was dumb, because I did not QT the Tangs when I bought them. I know I should have because of the condition the Yellow Tang was in, but I didn't have a QT tank. So I did put my other fish at risk, which I shouldn't have done. But, thankfully it did not back fire on me..
Also I don't think all of that guys fins will grow back, he looks like he is down to the base on some. If you can get him back to good health he will regrow a lot of the fins back, but he will be permanently scared. I am lucky enough that my tang had no base damage.