Welcome. I recommend following the advice on this website:
Correct Savannah Monitor Care
I put my savannah monitor in his adult enclosure (4'x8'x4') when he was a couple months old, so I'd get started on your enclosure right away. Also, make sure that if your floor isn't immediately on a concrete foundation that your floor trusses can handle 2000 lbs in a 4'x8' footprint. Many can, but some require modifications by contractors.