The rule for Brevs is 10 gallons per animal. I imagine you could go smaller, but the females might suffer if the male is the amorous type. Here is a link to a site for converting inches into gallons:
http://www.fishlore.com/ConversionCalculator.htm
Hope all goes well! Cheers!
P.S. I have two clutches of Brev eggs incubating now. I'm excited to set up a new viv for the next generation! :0)