Width: If measuring the u-joint, 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 u-joint.
Cap Diameter: If measuring the u-joint, simply measure the u-joint cap. If you are measuring the end yoke, measure the width of the opening where the cap sits in the yoke.