80sq.ft can be the minimum size
The minimum bedroom size depends upon in which country you are residing. The proportion of the apartment is also very important. For example, 6000 sq feet house would have big bedroom than a 3000 sq feet house. The standard are set in every country according to uniform building code.
From what I can find online 70 sq ft min, with at least 1 dimension being 8 ft, but each area differs, and the year built the codes could have been different