in Gurps, the Modular Cutter book (really cool resource) has them at 21' x 21' x 42' (21 foot diameter, 42 feet long, close enough to 6m diameter & 12m long = (err, pi * r^2 * l = 3.14 * (3*3) * 12 = 339.12, divide by 13.5 tons/m^3 = 25.5dTon; divied by 14 = 24.2 dTon)
Note: 21 feet = 6.4m actually. I'd just do 6 meter diamter x 14.5m long, giving 29.26dTon.
Edit: this just for the module itself, not the cutter portion.