If the default units of your standard.ipt file are mm, any mirrored part will be created in mm. You can change this default by finding the standard.ipt file in windows explorer, openning it, editing the units and then saving the file. Only do this if you want any new parts to be created in something other than mm.

If you just want to change the file you are working on, click on "tools" in the tool bar, then select "document settings", and change the "length" units.