Here's my 3d-printed Land Dreadnought painted up and ready to roll. It's nominally in 6mm scale, but it's big enough to make an appearance in 15mm games too — the model is about 65mm long.
It's been printed in BSF sintered nylon, of which I am not particularly enamoured, and has had a couple of coats of acrylic gesso (among other things) in an attempt to subdue its very pronounced surface texture.
From Facebook, copied here so I can find it again:
In 1/300 scale, that howitzer would be something on the order of a 500-600mm gun, and the thing would have a turning circle of about half a kilometre.
I'm thinking that it could form the basis of an objective game: one side is defending a city, the other is attacking it with this monster (and other much tinier ground forces).
The game starts with the defenders in control of the high ground, so the attackers have no direct observation of fall of shot into the city. The side with the Behemoth win if they manage to take out a specific city block; the other side win if they can capture or destroy the Behemoth. The Behemoth fires blind once every turn (couple of turns?), so I guess it's possible they could get a win by sheer luck. For the sake of balance, there should be some equally unlikely way the defenders could luck into a win — the terrain for the battle is underlain by layers of limestone, gypsum and anhydrites, and so there is a non-zero chance each time it fires of the Behemoth being swallowed by a gigantic sinkhole.
The attackers have to get the high ground to be able to direct fire accurately AND defend the Behemoth, the defenders have to keep the high ground AND attack the Behemoth.