Alpine suggests the following:

Gross Volume: 1.3ft^3
Sq Area of port: 11in^2 (11"x1")
Port Length: 22in
Vent Displacement: .287ft^3
Driver Displacement: .050ft^3
Net Volume: 1ft^3
Dimensions: 12.5"H x 14.5"D x 17.25"W

I say build a box thats around 4-5ft^3 tune to the kind of music you listen to and go from there.

37.5"W x 14"H x 20"D (external)
36"W x 12.5"H x 18.5"D (internal) (4.818ft^3 Gross)