I have a sheet with different "box" sizes. input via a drop down list.
Dimension a Dimension b
1100 1200
600 800
800 1000

Then a pick list which gives either SHORT side or LONG side leading.

The user picks one off the 1st list e.g. 1100 1200
If they pick SHORT the form should fill in two boxes
Width 1200
Length 1100
But if the user picks LONG, the auto fill should swap the two dimensions
Width 1100
Length 1200