I’m trying to create a tag for a space that’s 6 x 9 mm
Check the menu Tools > Options then select the module (LO Writer/Calc/Draw…) then General. The Metrics section will let you change the unit.
Note that your tag is strange: Math is for the formula module. Did you mean Calc? Or Draw?
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Note that you may also use alternative units in the size controls even without switching to the other units globally. E.g., using inches as your main unit, you may type 6mm into the size box (explicitly stating the units), and it will be automatically converted to 0.24". That may be useful when you only occasionally use alternative units.