Depends on which Tristis you are talking about.
I have tristis orientalis.
4 yo, 40cm total length.
A trio live happily in a 4 x 2 x 2 tank with enough room to hide, explore & bask.
Adding ledges increases the usable internal area also.
A Tristis tristis would definately not live in that size tank for long.
So there you have it snakemadness.
There are a lot of other factors to be taken into consideration before you settle on their housing requirements.
Do a bit of reasearch into what is available from where & see how each individual breeder houses them.
If you house them similar to the breeder does, you can't go too far wrong.
I've raised dragons under these lights with no problems. As with all lizards I still take them outdoors once a week or once a fortnight for a basking oppourtunity in the sun when weather suits. Monitors would thrive under these lights I have no doubt. I did raise one clutch of Varanus tristis tristis under these for a short amount of time with no dramas, but the entire clutch was sold quickly so not sure what the results would be long term. I'd say fine - just watch room temps in the summer that you don't cook the lizards.