It can be difficult to size the earlyish ones on sight, because both the seat tubes and the head tubes were longer than in later years.
The owner says it's a size 21, not an unreasonable deduction from the seat tube c-t being 53.5cm. However I think it means that it's a size 20 (the length of the seat tube itself, not measured from the centre of the bb).
Size 20 would also be consistent with the length of the head tube, which appears to be 17cm (up to 1992, the head tubes were 2cm longer than in later years).