I had a Coral Beauty in my 40 breeder. The 40 breeder actually has a pretty good foot print for overall size but it does depend on how full of rock and corals it is. My CB was on the smaller size and probably the bigger fish in the tank. Only other fish was a pair of pajama cardinals, pair of clowns, Azure damsel, and lawnmower blenny. It wasn't packed with rock but I did have 4 rather large hammer corals and about 6 rose bubble tip anemones. Probably the only reason why I picked it up was due to the size and I knew I was moving the tank into a 210 gallon that was finishing up its cycle. I will say while it was in the 40 it was fine and never looked out of place. He did wreck one of my hammers but not by eating - more so by living in and using it for a bed. It picked at the rocks, didn't nip any corals, ate just about everything, fast swimmer, constantly moving, etc.
I also use LA for fish sizing but use it has a reference. At the end of the day for me I knew the 40 was temporary but it did work. It was a tough tank to fill because of the foot print and how it can be misleading. If I didn't have an upgrade in the works at the time I probably would have passed on it if truth be told.