You don't realy nead to measure the width of the disk, its more important to check that the rivets are below the level of the wearing surface on the lining. When a disk is worn out the flywheel or presure plate or both will rub on the rivet heads causing scoring of the iron components, as long as the rivet head is below that level by say about .025" then it will be fine. Also check that the cussion springs are firm in the disk so they don't spin or rattle around, (just grip them with your fingers and see if they will move around, if they do you may need to replace the disk) and check the wafer springs between the linings to see that they arn't breaking up and causing a high spot.(IE There is a set of springs that actualy hold the two halves of the lining apart, and sometimes they can fatigue over time and start to break up, if that happens it's a new disk time). Unless its been raced or driven realy hard thats unlikely. Also look for oil impregnation, if a rear main or a gearbox first motion shaft seal has been leaking onto the lining replace it or it will most likely slip under power or shudder like fury.
Goodluck