I am very bad with remembering sizes. It could have very well been 6mm. I am often quite annoyed with myself over this. I think I only put the comment of 12mm there to make sure that it was at the lower end of the large larger range I picked. I should have picked the smaller range, I think.
Monolepta minima as identified in the ANIC (I have just looked at them) has a consistently different pattern. I can't find any identified specimens that match this one, so I will call it just 'Monolepta sp.'.