When I initially made my Place Shapes Toolbar extension for SketchUp, I created it with a weird unit convention that used one foot as base size in the US and one meter everywhere else. That did not make much sense at all, because even in the US, folks might want to use millimeter-sized shapes for 3D printing modeling. Fortunately, with version 1.1 of this extension, this practice is gone!
You can now select a base unit from a convenient drop-down and all shapes will be sized based on that unit. You can even switch units while modeling and already placed shapes remain as they are. Only subsequently-placed shapes are scaled to the new unit. That should be much more usable then the old method.