According to this federal study (the link downloads the PDF) most German speaking cantons use the formula given above, number of rooms plus one is the maximum persons (with leeway for large families in many cantons).
However Basel-Land (BL) and Basel-Stadt (BS) are the two exceptions. Their rules are more complex (see page 30) but a 2.5 room apartment is fine for a couple without children.
The requirement is a snapshot at the time of application. Nothing keeps the applicant from moving to a smaller apartment later on.