SYNCHRO plugin for Navisworks only supports meters as a unit of length and converts every other unit supported by Navisworks to meters, including some of the parameters exported as user fields (Area/ Volume/etc). This was designed this way so as to deal with possible multiple models each with their own internal units appended in Navisworks. However, you can use the following workaround -

1. Default Units setup in Navisworks was Feet, where the value = 66.736

2. Exporting this using Synchro Plugin converts this value to Meters = 6.20

3. Change the units of the model (right-click on model >Units & Transform) and change this from Feet to Meters, which changes the value = 718.343. This is a temporary step so undo the unit transformation after exporting as SPX.

4. Update the Synchro model and you will notice the value actually changes to the initial Feet = 66.74. Now you can change the Units from Meters (default) to Feet, from under Navigator Ribbon