It'll largely depend on the abilities of the particular artist. I'd think that most would want to start at least half again as big as what I'm seeing here.
What went wrong was the the tattooer was in WAY over his head on this tattoo and tried to do it anyway. It doesn't look unfixable, it's just going to take some time, money and maybe the aid of a laser.
Where do you live, maybe someone on here could give you a recommendation?
Portraits are incredibly hard to do well and very few people do them really good. I would NOT go back to the same artist to try to get it fixed. I think your only hope is to find someone that specializes in portraits and coverups and see what they can do.