The healthiest approach from my perspective, as already mentioned, is to keep your feet on the ground. The times of moving from a 2.5-room apartment into a 180 sqm palace with low interest rates and then hypothecating it down to the chimney with the corresponding creditworthiness are over. Will those times come back? No idea.
Statements like: a children's room under 15 sqm is not a children's room, are history. Over the years, there have always been trends that then disappeared again. Right now, home office is very popular; in the past, it was the living room with the Dallas open staircase. So the question is, do you have to follow every trend?
Kitchens have become something to show off today, energy-saving technology not even counting photovoltaics. I would make sure that the house causes little follow-up costs from an energy perspective, because energy will certainly not get cheaper. Paired with a reasonable size, yes, and for the small family I mean the mentioned 120 sqm.
Maybe plan it so, if the property allows it, that you can add on later or do an attic conversion, there are many possibilities. Of course, also consider the question of personal labor, does the lawn really have to be sod, and so on and so forth.