Choose a size that will comfortably fit the equipment you will use, plus leave room for expansion. I use a 40g breeder with my 75g tank and it's more than enough for me right now. It's way cheaper to build your own with a 40g breeder and some partitions!
Like Idoc, I built my first one, a 40B which worked quite well.
For the system I’m currently building that is a 100g, I went with a Trigger Systems Sapphire 36.
I personally like the Trigger System sumps because the baffle layouts are much simpler than others I’ve seen. The sump is there to hold equipment and all those little compartments in some designs can make selecting equipment a pain.
I too believe the 40B makes an excellent sump. However, here's my story. I purchased what was sold as a 40B; used it for years; wanted to do a rebuild and puchased another 40B. This is when I found out what I thought was a 40B was a 50B. Same footprint, but slightly taller. So my thought-to-be 40B is a 50B: