As far as I know, Miyata’s mixtes only came in two sizes, 19-1/2” and 21”, and the former are far more common. Like 1 in 20 might be 21”. This bike might be one of those but it’s also very possible the seller is measuring wrong.
The larger size would probably not be a good fit for someone 5’4”.
The first vintage bike I bought was a 1986 Miyata 110 mixte, for my daughter to take to college. Cb400bill facilitated that purchase as the bike was near him over in Michigan. Bike was in mint condition and daughter still has it, 12 years later.
So I’m a little prejudiced in saying that you almost can’t go wrong with a Miyata mixte, and I look forward to seeing pictures of this one.