To measure a door cylinder when it's inside a door, do the following. Measure from the centre screw hole (seen in the red circle) to the edge of handle backplate on the outside and round up the size to the next 5mm increment. E.g. 33mm would be 35mm.
Doing this will ensure the cylinder protrudes from the backplate of the handle.
Now do exactly the same and measure the inside.
Note: The screw hole is the retaining screw for the lock cylinder
If you are ordering a new door handle be sure to allow for any difference in the backplate thickness.