You can keep a baby or juvenile monitor for a little bit in there, until you need to move it into a larger cage. But most of the larger monitors are a challenge to keep (even the medium-sized Indo monitors are considered some of the most challenging captive Varanids.)
Ackies are always a great choice for a beginner varanid keeper, such as myself
They would need at least a 4 foot by 2 foot once they reach they're adult length. You could keep other even smaller dwarf monitors in there for life, like Storr's, King's Pygmy, or Pygmy Mulga (all of which stay roughly around a foot long!) But the later ones could cost $1000 a piece! I doubt you have that kind of money laying around or would want to spend that much on a lizard, but I could be wrong!
Good luck!
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Jason