If you already have the dragon, I would have use that as the starting point for deciding on the size of the opening.
Six inches seems like a good overall choice. That would scale to aprox 50 feet (or 15m) with 15mm figures. That’s a very large door/gateway, but seems to fit with how Tolkien described dwarven architecture, with enormous halls carved within thaw mountain. A little ironic maybe, given the short stature of the original inhabitants, but if you have the technical ability, why not flaunt it? Of course, if they made the gate and halls smaller, they might not have had a dragon infestation in the first place…
Anyway, a monumental gateway definitely seems like a cool idea!