Yeah, that's a concern considering Lord of the Rings models are 25mm scale were smaller even without 20 years of Warhammer scale creep. From the last Warhammer Community article a few days ago:
You can see a Gondorian soldier right next to the tower's door. I think that one is leaning forward, so it's a bit misleading. But a new Cadian might just pass through that door without ducking. So if your standard is previous metal Guardsmen and normal sized modern plastic humans (Genestealer DNA optional), the buildings might work decently well. Perhaps the windows and balcony wall are a little low, though.
Worst case scenario, the buildings can be used for the Ratling district.
The thing about
40k terrain is that it's scaled to look good even with huge Marine models next to it. That is something you will definitely lose with the Gondor terrain. It's strictly built with normal humans in mind. Transhumans need not apply.