JV48PC Ball Shape Sprung Mortice Knob

Standard size is a 2.5 inch / 63mm latch or lock or a 3 inch / 75mm latch or lock should be fine.

The length of the Lock and Latch determines the Backset which is the distance from the edge of the door to the centre of the Backplate on the door handle where the spindle/bar passes through the door.

The Backseton a Standard 63mm Latch or Sash Lock is approx.45mm and on a 75mm latch or lock is approx.57mm.