Plot of land 371 sqm, are the expectations realistic?

Well, that really depends on the importance of a garden for you personally. We currently have around 100 m² of terrace and 200 m² of garden (lawn, with a garden shed, enclosed by a hedge). I wouldn’t have wanted more, but I do find the large terrace great. Some acquaintances always shake their heads because of the sprawling terrace, but priorities are just that different.

Such "problems" are also gladly taken away by the development plan through the specification of floor area ratio / site coverage ratio
Edit: Building lines/boundaries can also impose restrictions.

The zoning plan will probably be crucial. We will have 30x18.8m. Building boundary all around 3m. Building volume will be 12x7.5m. I am convinced that something nice can also come out of 20x18.8.

As already mentioned, an evaluation is only possible with location and zoning plan.

Such "problems" are often taken away by the development plan through the specification of floor area ratio / site coverage ratio.
Edit: Building lines/boundaries can also impose restrictions.

The decisive factor will probably be the development plan. We will have 30x18.8m. Building limit all around 3m. Building volume will be 12x7.5m. I am convinced that something nice can come out of 20x18.8 as well.

As already mentioned, an assessment is only possible with location and development plan.

That is the "problem," our development plan is quite "loose" and doesn't take anything away from us .. We have a site coverage ratio of 0.35 and a floor area ratio of 0.70 WA II/O otherwise no requirements..

edit: 3 m boundary distance

we are so undecided whether one should build as small as possible due to the plot size or only as big as necessary, or whether one should incorporate all their wishes and then have a smaller garden..

With a summer house, a slab is enough. A slab without a summer house seems silly to me.

may I ask what dimensions your house will have? we also have "only" 389sqm of land and are currently endlessly browsing floor plans. we are unsure whether, due to the size of the plot, we should build as small as possible or only as large as necessary, or whether we should incorporate all our wishes and have a smaller garden in return..

We built as large as possible (no development plan, but restrictive building laws in Bavaria that unfortunately make infill development quite unattractive). The dimensions are 12.5m x 7.3m. The garden will still be about 70sqm, which is enough for us due to lack of a green thumb. Besides a grill with a seating area, a paddling pool, and a sandbox, we don't need anything else. But of course, that is very individual. We are building without a basement, so we definitely need the space in the house.