I'd suggest having 2 input fields, one for feet and one for metric. Then do the following:
For the feet field, make sure the field is NOT required.
For the feet field, check the "has depending fields" box.
For the feet text input, check the "depends on another field" box.
For the feet text input, set the enabled property equal to a macro of
FeetMacroDropDownFieldName.Value == 'feet'.
For the feet text input, set the validation rule = maximum value and set your max value.
Duplicate the same for the metric field.
Doing it this way simplifies things a bit so you don't have to try to do it all in one field.