It all depends on the baffles, and the amount of area that is left above your regular water level for the overflow in a power outage. I have seen 20longs under 125s, and thought wow...thats not good....I have a megaflow 4, Allglass sump (POS btw) its their largest sump under a 120, and think its a bit small, but its smaller than a 30 gallon.....I am sure it will be fine, but with sumps and other things in life, bigger is better
An AGA 150 has a 72x18 footprint. Every 1" of water is 5.6 gallons. As an example, if your display level drops 2" when you turn the return pump off, 11.2G of water will drain back to the sump. Add in what drains out of the return and drain pipes. You have to be sure there is enough reserve capacity in the sump to handle the drain back.
If you want to use a sump under the stand based on a standard aquarium, I'd use a 55G.