Because it has a place for a doorbell and name, my letterbox is 15" long. I have a non-wired doorbell on the wall now but there is a hole behind the door plate which will need to be covered with the new letterbox. All the double glazing including the front door was carried out by Stevenage Windows, Knebworth who are not there now, about 20 years ago.
you wont be able to buy a lettrbox at this size,you would have to replace the panel in your door if it has one,if letterbox is in a midrail the only way you could fit one is to buy some upvc trim and trim round it to hide hole.hope this helps