hey, im thinking of getting a standing day gecko in the future and have been looking up on what they require, however im unsure on the size of the viv i will need, i found a care sheet that told me they need at least a 20 gallon tank but i dont know what tis means and i dont think pet sops witll appretiate me walking in with 20 measured gallons of water and pooring it in a variety of vivs

can anyone give me some actual measurements and save the poor petsop from me and my water measuring madness?