The largest glass tank you can give them to be honest. I kept two gerbils at any one time in a 30" inch long aquarium with a mesh lid (no tank topper) as they tend to chew the bars and injure their teeth. I would never put gerbils near bars for this reason. It`s not worth risking injury. I`ve been there and it`s not good. Anything smaller could prove restricting once you put their next box in it of a wooden platform etc...
I`ve seen gerbils being kept in small tanks and thy really do appreciate space. As Drago said, they love to dig and are burrowing animals. I call them mini-meerkats!
Males tolerate one another better than females, but two females can live together if they are related. I always kept males and had four gerbils split into pairs because of a big de-clan. I wouldn`t keep more than two together because of the de-clanning risk.