Actually, there are mixte frames in 56. Sometimes, even larger. They are rare, of course, but they do exist.

Many of the smaller frames have longer (virtual) top-tubes than the seat tube length would suggest. This frame, for example, although a 49cm, has a 53cm top-tube.

Still a tad smaller than you might want, but still more generous than you might think. This is true of the classic mixte design in general.

People keep nibbling, but no one's made an actual offer.

Please don't make me take it to E-prey!