The RTS gets up to 7 inches, here's mine. Not a very good pic, but really is that big. Almost as long as the width of notebook paper. Also appx 4.5" tall and about 2" wide, I think female due to her shape and lighter color. She was given to me full size and the person had her since a baby about 7 years old now.
She was temporarily in a 32G while she was in quarantine and for a little longer than the quarantine time while I set up my 72G and got it running properly. 32G was really too small.
A 55G tank is only 12" deep, so maybe a little larger tank would be a better choice. Also make sure there are cool areas around the tank bottom for them to hide in. Mine has a favorite tree root deco that was in my quarantine tank and I kept it for her so she would be happier in the tank transition. They want a place they can zip into and out of which would be even a 3/4 hollow log type thing/ "rock" cave.
They can be aggressive especially if there is a smallish tank and they feel like they have to protect their territory. Mine is pretty mellow but would chase the other bottom fish out of her swim path just to show who was boss.