Width: If measuring the ujoint, compress the caps and measure from outside of cap to outside of cap. If measuring an end yoke, measure from inside of tab to inside of tab. If you do not see a tab as shown, then you have an inside snap ring ujoint.
Cap Diameter: If measuring the ujoint, simply measure the ujoint cap. If you are measuring the end yoke, measure the width of the opening where the cap sits in the yoke.