Therefore I need a 6mm DropShip and a 28mm Scale one.
So with a bit of judicious scaling, and simplifying of a number of features that would be lost at 6mm scale, this is where I have got to so far. I have the tails to add, and a little more simple detail on the hull (a couple of vents, access hatches that kind of thing).
Overall dimensions 52 x 34 x 17
I think this will have to be cast in two pieces. The hull and engines should be cast in a single piece (though I shall have to smooth the join between engine and hull a little). The cockpit split line is not coincident with the main hull split line so I suspect it will have to be cast separately.
The tails are not shown here - but I am hoping they would be strong enough to be cast with the hull... they'd be very fiddly if cast separately.