That's twice the size that current wisdom had suggested. A well-worn complaint about italic turned out to be correct. The letters m, n, and u are too similar. They tend to turn into a scribble. The looks and presentation of the teaching materials made greater difference than I expected.
Education is an odd collaboration and, when other things are right, seems to involve magic.
It helps if the children like the copybook, and helps again if the teacher likes it as well. At the moment, we're overhauling the handwriting project. Revised teaching aids in two parts will be introduced soon. One will be a basic set of instruction books. The other part will be batches of exercise sheets.