I have a friend that had a sand sifter that was about 4-5 inches across in a 10 gallon tank with VERY coarse sand and it lived for several years. I think the reason he died was the type of sand, honestly. There was also a TON of rock in this tank. It was stacked top to bottom, front to back.
I've also got a 42 gallon hex tank (very small surface area vs. volume) and I have 2 sand sifting stars... one's about 6 inches across and the other's about 4. They've been with us for a pretty good while so far.
92-gallon corner tank, 100lbs of LR
, 140lbs of sand, 250watt 10,000K MH
, 110watts of actinic PCs, Mag 7 return, custom refugium, AquaC EV180 w/ Mag 5
Female lyretail anthias, eyelash blenny, tomato clown, saddleback clown, firefish goby, 2 sand-sifting stars