That a good question!!!
When I build my viv's I researched for the same answer but never found a good one. I couldn't find a ..."square inch of vent needed per square foot of viv space"  to refer too. 
My viv are 25"x19"x18" and I have two vent on top that measures 3"x20" 
and I never had a problem. 
I would say just plan on adding more vent it needed.