with Rainbow crabs, the key is not to have plural. Just a single.
Mine lived for 11 years!!! No heater, no filter. 80cm tank split by a wall, one half was land mass (sand and small pebbles mixed) and the other side water. I just used water that was coming out of a fish tank.
Dont bother with any plants, real or plastic.
Very entertaining creature, and became rather large also. Plus 1 on the escaping... they can scale virtually anything, and if they can stretch yo get the tip of their foot onto something, they will be able to (eventually) clamber up it. They seem to spend a bit of time thinking about how they can get out to explore.
I'd say 80cm is about adequate, but if you can go bigger then do so.
Easy to look after. Change the water every now and then. They live on the land. Occasionally, you'll want to give the land area a good deep rinse too.