I think 'weitläufig' is the closest so far, though it's not ideal.
For me huge would be an overstatement. I don't think of a rambling house as a mansion or anything, just one that's spread out in a wandering, unpredictable way, with interesting nooks and turns. One that's built generally out rather than up.
In (historical) Bruges, though, where the medieval houses are tall and close together, I would guess it could refer to wings, additions, staircases, etc. built on at odd angles later than the original core of the house, or perhaps to two or more smaller houses that were later joined together, creating some rather meandering hallways or unpredictable room layouts.
Sorry, Werner, this has nothing to do with the term 'rambling house' as a fixed two-word designation in historical British usage. I had never heard of that, and it's an interesting little piece of information, but here the word 'rambling' just describes 'house,' as is usually the case.