Hi Justin,
There are several easy ways how to archive it – for example, by adding a Transform Operator between the Segment and the Plank Floor Macro, exporting the X Fixed Size Parameter and driving it by a Random Parameter where you can easily adjust the Range in Scene Units. Just please take note, that it should be set to Generate on: Segment. That should do the trick.
Optionally, you can just enable under the Transform tab of the used Segment the Random X Scale and adjust the desired percentage.
Best regards,