Alex Garner of Tannoy in Scotland made the following recommendation 
for the 12" Monitor Gold. 

Compact with response to 48Hz- 75 Litres, 2 x 50mm diameter ports, 
approx 100mm long.  Ideally, tune the minimum between the two impedance 
peaks to 48Hz. 

Ideal with response to 35Hz- 150 litres, 2 x 100mm diameter ports, 
approx 200mm long.  Ideally, tune the minimum between the two impedance 
peaks to 35Hz.

For 15" monitor gold he recommended 300L with 4 ports of 100mm 
diameter, 250mm long.  That is what I have and am very pleased with.

Peter Campbell
Australia