Because of the 2 flame angels, 75-100gal. Because of them copled with the mandarin dragonet, I would say 75-120gal. I wouldnt go lower than that with the dragonet because it needs the larger volume to upkeep its specific food requirements. As always with marine aquariums, bigger is always better.
the biggest one of them i've ever seen was like 2 inches... i thought they stayed like that?
i have a goldhead sleeper goby in a 29, hes about 4 inches right now. honestly if it's the only goby (the twin spots do better in pairs BTW) and there's enough sand, cause that species is a filter-er, i think they'd be fine in smaller tanks