I give my cresties at least twice as much room as "recommended" on the 'nets, and they seem to do better for it. Frankly I consider a 20G high turned on end to be a standard volume to house a single gecko.

I used to house two geckos in a 2'x18''x2' and that was a great size for a pair.

However I have since moved all my crested geckos into single-living, only pairing up temporarily for a few days at a time for breeding purposes. This helps me monitor food intake/output much better.