It is important to take care with the sizing to ensure your koki fits firmly and doesn't fall off during The Play. This can be embarrassing and can inhibit the natural flow of action and dialogue.
For accurate sizing the organ should be firmly pressed against the screen image. As this creates a rather difficult viewing angle, it is a good idea to get your partner to check your reading. Measure the size at the thickest part (usually the stem) by holding a flat vertical object like a ruler against the organ's edge and lining it up with the appropriate line.
Alternatively, print this page, cut out the black circles and test for a comfortable fit.
Having found your width (eg., B+, C-, etc), you can cut out the two corresponding slits marked on the holding tabs and be assured of a good fit. Remember to cut only the slits for your size so as not to weaken the tabs with too many cuts.