To reproduce:
1. Open attached project;
2. Open "Example" scene;
3. Select TextMesh in Hierarchy;
4. In Inspector window, try to change Font Size to 0.

Expected result: it is possible to change Font Size back to 0 once it was changed (when set to 0, editor uses Default font size).
Actual result: it is impossible to change Font Size to 0 once it was changed.

Note: It happens both on dynamic and non-dynamic fonts.

Reproduced on versions: 5.4.4p3, 5.5.1p1, 5.6.0b9.