sump size...?

[Chaloupa]

Feeling like a newbie here but......is there a rule of thumb for sump size to tank size? I've always had an Oceanic sump for most of my tanks...I have a custom sump for a couple but just because they worked out well for the tanks I have, and have never really thought about it.

Tank in question is a 6 foot 210g....AND where to get "said" sump? I see the Eshopps sumps but boy are they small.....and am just a bit unsure. Would you have a custom sump built or would you just get an "off the shelf".

PLEASE give me your input and advice....I thank you in advance!!