Welcome to LJ, rickanthony,
I suspect that the Wen bandsaw will struggle cutting a 6" tall bandsaw box. I used to have an even smaller 9" Craftsman with a 2.5A motor and it would slow down cutting 1.5" thick 2×4s. It could still cut if you go slowly. I typically used it for small objects like 1/4" plywood and it was great at that.
The Wen has 2 advantages over my saw. The motor is 3.5A (1/3HP) instead of 2.5A (1/4HP). And it has a 2 speed pulley that would give it a 40% mechanical advantage at low speed. Both of these factors combined should give it twice as much power as my saw. This could allow it to cut reasonably well in the 3" range. 6" seems like a stretch. The lack of power might be the reason they added the low speed range.
You could still make 3" bandsaw boxes.
I suspect that the Wen bandsaw will struggle cutting a 6" tall bandsaw box. I used to have an even smaller 9" Craftsman with a 2.5A motor and it would slow down cutting 1.5" thick 2×4s. It could still cut if you go slowly. I typically used it for small objects like 1/4" plywood and it was great at that.
The Wen has 2 advantages over my saw. The motor is 3.5A (1/3HP) instead of 2.5A (1/4HP). And it has a 2 speed pulley that would give it a 40% mechanical advantage at low speed. Both of these factors combined should give it twice as much power as my saw. This could allow it to cut reasonably well in the 3" range. 6" seems like a stretch. The lack of power might be the reason they added the low speed range.
You could still make 3" bandsaw boxes.