What size tank would be best for it long term? I was thinking of using a spare 46 gallon bowfront in my possession as a new tank. Would going bigger be a better idea instead of the 46 gallon bowfront?
Going bigger is always a better idea. You may be able to get by with the 46 for quite some time though.
In theory, these corals should be able to outgrow most of our tanks. In the wild, where growth rates are stunted by seasonal changes, and other environmental influences, these corals can reach several feet across. In our tanks, we can eliminate factors that reduce growth rates. We can provide environments with no cloudy days to rob energy from the coral, no rainy season to reduce salinity, all the food the coral can eat, no predators, relatively constant temperatures, no pollution, and I'm sure I'm leaving a few off. Basically, we should be able to do with these animals what we've done with captive tigers. With medical care, great diets, and good living conditions, we've created tigers that live longer, grow larger, and are better athletes than their wild relatives. I see no reason why we can't do this with elegance corals.
With most of the elegance corals I've seen offered for sale in the hobby, their growth rate, or calcium carbonate depositing rate, in the wild was somewhere around 1/4" to 3/4" per year. I've personally obtained growth rates with these corals in captivity of around 1-1/4" per year, and I'm sure that's not their maximum growth rate.
HTH
Peace
EC