i run a 300 display and a 110 sump so with your 180 you can do a 60-75 easily just make sure you have enough space under your stand to work in your sump
How big the sump is, should be a matter of how much room you have, not how big the DT is. If you are going to do a sump or a sump/refugium you may as well go as big as you can. The whole idea is to have more room for filtration and more water volume. I have a 180g and 70g hexagon sitting next to each other on a 10' stand and an 8' x 2' x 18" (180g) sump/refugium/DSB. It's an old frag tank from an LFS who got new tanks. Here is what it looked like as the DT's and sump were fitted into the basic stand frame.
100% agree with Mr. Reefman. I'm only using a 55 gallon for my sump because it was all I could fit in my stand (I way overbuilt my stand but that is another topic). Go as big as you can.