The way the drop dows behave (in regards to be disabled etc) is not ideal.

For instance, for the Header 1 element, create a Box->width element

The property has three values:
numeric value (text field)
auto/inherit (drop down)
unit(px, ems etc) (drop down)

If you enter a value in the text field, the auto/inherit dropdown is disabled
However, if you then clear the text field, the auto/inhert dropdown remains disabled. You have to close the property dialog and reopen it in order to make that drop down enabled again.

Also, if you enter 50 in the text field and select "px" as unit.
If you then close the property dialog and reopen it, you'll see that the value of the text fields is "50px" (not 50 as expected) + the unit dropdown is disabled