If you go to Reef Central's home page they have a calculator for determining sump size. The calculator is more about flow through and drain off from your tanks should the main pump shut down. What it doesn't do is calculate floor space for things like skimmers and if you want a refugium.
The bigger is better approach comes from wanting more water volume for the entire system. I believe this is a good thing as it results in a larger more stable body of water without adding any additional bio-load.
Personally, I have (or will have shortly) a 200 gallon display tank, a 60 gallon frag tank, a 30 gallon isolation tank and a 10 gallon fuge all plumbed through a 40 gallon breader as a sump. I have the sump divided into three sections: an 8" return pump section, 6" for four baffles and the remaining 22" for the skimmer. I actually use an external pump so have some leftover room in the return pump section if I want to add an internal pump to supply reactors. Other than that, I'm not sure what else I would need space for.
What I want in a sump is something that gives me adequate space and is serviceable. In my opinion, a 55 is too narrow and limits the number of skimmers you can use. I like the foot print of a 75, but it is a little too tall as I don't like to have to dig that deep into it. The 75 also doesn't fit under a lot of stands and if it does, most stands are 30-32" high with 3 -4" face frames which only leaves you 6-7" of access to the sump. My ideal sump would be a 120 the height of a 40 gallon breader; 2' x 4' and only 16" high.
