Hi, Jack:  I have never had a problem with discs of any size.
I once had a piano roll rejected and had to pay additional postage,
which I subsequently got back after showing the Postmaster the
actual DMM rule (Domestic Mail Manual) Section 473, subsection 2.2e.

You might try to convince her that musical box discs are the same
as piano rolls in that they contain the program of music that gets
rendered on a musical box in the same way that a piano roll's program
gets rendered on a player piano.

Attached is the section of the DMM that I refer to.

Best wishes,
Bill Wineburgh