The largest sump you can fit under your stand or in the place you are considering for it is the best advice. The larger the sump, the more total water volume, more space for your protein skimmer and reactors or refugium or whatever you want in it. As far as the tank size, Some of that also depends on which tangs you are considering.
If this is to be a reef tank, I would suggest a 125 or 180 over a 150. the reason is, a 125 is not as tall, thus easier to reach the bottom. The 180 is 24" front to back, which is much better than the narrow 18" f2b the 150 offers, plus, it's also 4" shallower, which, like the 125, will be easier to reach the bottom.