They won't reach closer to adult size until 2-3 years down the line, but I'm currently thinking a 4x4x2 foot AP cage! So 4 feet length and 4 feet tall. Or a custom glass cage around that size.
For now, he can live in this 15qt for at least 6 months, and I'll just upgrade tub size. The biggest sterilite tub is the 110qt and that would also fit an older one comfortable for quite awhile.