Great post, Saskia.
I also wanted to add this. You can keep a single horse on an acre, but doing it well and providing good care is both expensive and labor intensive. The more acreage, the less labor intensive and expensive it is. For example, on a smaller acreage, all your forage will have to be brought in and you'll probably have to supplement with concentrates. You will need to keep the horse off the paddock at least 50% of the time, which means buying bedding and mucking daily. You will also need to pick manure in the paddock, and fertilize and reseed the paddock regularly.
Larger acreage, the grazing replaces much of the purchased forage, horse can stay out 24/7 when the weather's fine so less purchased bedding and mucking, you can manage the paddocks by dragging and rotation and fertilizing and reseeding does not have to be as intensive.