Steps to reproduce:
1. Open attached project.
2. In the "Project" window select asset named "New Font".
3. In the "Inspector" window extend "Character Rects" option, click on "Size" field and enter any new value, for instance, "23".
4. Click somewhere outside of the field.

Actual result:
The value of the field is being reverted to its previous value.

Expected result:
The new value of the field should remain after clicking somewhere outside of the field as it remains when changing the value of other fields, such as "Ascii Start Offset", "Tracking", "Line Spacing", etc.

Reproduced with:
5.1.0f3, 5.2.4f1, 5.3.4f1, 5.3.5f1, 5.3.5p1, 5.4.0b18.

Workaround:
1. After entering the new value press "Enter" key.
2. After entering the new value press not somewhere randomly, but on the other field, for example, on "Font Rendering Mode".

Note1:
This issue is reproducible with any object that has a field to set elements size, for instance "New Flare", "Button" (in the "Debug" window), etc.

Note2:
GIF attached.