Replacing garage door lock barrel
The lock barrel is known as a Euro cylinder (there's a photo on this page showing Euro cylinder locks booth side and end on).

The numbers XX/YY relate to dimensions (in millimetres) of the lock when viewed from the side. The dimensions are measured from one face of the lock to the fixing hole in the centre, then from the hole to the other face.

You can remove the lock, measure it, and replace it while you wait for the new one to arrive - it only takes a matter of seconds with the key to remove the cylinder. However, the bad news for you, since you say that you've not got a key, is that you generally need a key to be able to easily take the cylinder out. As you can see in the top of the photo on the link, the cam is slightly offset when the key is withdrawn, so it doesn't just slide out once you've removed the fixing bolt.