What size latches / locks should I choose?

The size of the latch is only critical if you are fitting new latches into existing doors and you need to make sure the main bore hole lines up with your new latches/locks on the face of the door (this is called the backset measurement). All measurements are shown on the product page along with line drawings to assist. On new doors, you can use 63mm or 76mm latches/locks, although the minimum should always be 75mm long if you are fitting door knobs, so as to reduce the risk of crunching knuckles as you turn the door knobs.