Revit calculates system units in Imperial units (Feet and Fractional Inches). When Metric units are set as the “Project Units”, Revit dynamically converts the Imperial units into Metric units, in the UI. This means that FBX always exports in the actual system units, Imperial (Feet). There are no issues in working with Metric units in Revit and 3ds Max, but this results in an Imperial (Feet) FBX file, that automatically converts to Metric (Meters) on FBX import into 3ds Max. When you use an Imperial (Feet) Project Unit in Revit and Feet System Units in 3ds Max, no conversion is needed.
For more on Revit units, see Why is my scene scaled differently in 3ds Max?.
