Start by measuring the height of the inside of the stand. Make sure that when you plan your sump you allow room to take off the skimmer collection cup and are able to "get into" the sump to clean. I'm using a 75 gallon tank that I siliconed some glass baffles into as a sump/refugium on my 180. It's a bit of a challenge to clean due to the limited space between the tank bottom and the sump top!. Good thing I've got skinny arms.
I also use 75G on a 180 but that depends whether your 150G is 18 or 24" wide. If it's 24" wide and 6' long then 75 is good to go. If your 150 is 18" wide and 6" I recommend 40G Long. But yea, like somebody already said, the bigger the better.
Can never go wrong with a nice big sump....just calculate the correct amount of water you will need to hold if return pump stops...can and will happen :splitspin..one other thing if you can try to avoid putting it underneath in a cramped area. It sucks trying to clean your sump in a narrow cramped undertank space.:irked: