that would depend on the type of plated lizard you are getting. Some plated species are MUCH smaller than others.
The little ornate plated lizards would probably do fine in a 20L, but a monster like my black-lined plated would need a 55 gallon or larger.

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~Sara~
5.5 ball pythons
1.0 Black-lined plated lizard (Lizzy Butt)
0.1 Burmese Python (Pixil)
2.1 Pet Rats
1.1 Dogs (Ozzie and Mandy)
0.1 Cat (Isis)
0.0.1 Synodontis Catfish (Big Spotty Fish)
0.2 Convict Cichlids