As anyone who's ever had to pound the streets posting things knows, letterboxes come in all styles and sizes. Big ones, small ones, low ones, loose ones, narrow ones, vertical ones, snappy ones... and some that are genuinely easy to fill.
Official guidance on letterboxdesign does exist. Alas not every front door follows it.
The original British standard was BS 2911:1974"Specification for letter plates", but this has since been superseded by European directive EN 13724:2013"Postal services. Apertures of private letter boxes and letter plates. Requirements and test methods". It's all very sensible stuff.
When determining the dimensions of an ideal letterbox, the key determinant was the C4 envelope. This is an envelope just big enough to contain an unfolded A4 sheet of paper, specifically measuring 324 mm × 229 mm. If you'd have to fold a C4 envelope to fit it in, your letterbox fails. When doing official tests the C4 envelope is actually stuffed with a stack of of paper 24mm thick, which marginally shrinks the envelope's length and width, but let's not worry about that.
Two different widths of letterbox are permitted, depending on whether the envelope is posted lengthways or side on. Assuming narrow edge first, the aperture has to be 230-280 millimetres wide. Assuming wider edge first, the aperture has to be 325-400 millimetres wide. In each case the range's minimum is just enough for an envelope to scrape through with 1mm to spare. An interesting corollary is that no letterbox should be ever be 300mm wide, or approximately one foot, officially at least.
As for the height of the letterbox, this should be somewhere between 30mm and 35mm. Any shorter and you might not get a parcel in. Any taller and security could be an issue. An additional standard concerns the dimensions of the space immediately behind the aperture which has to be sufficient to hold a 40mm stack of C4 envelopes. If your post drops onto the doormat it easily passes. But for anyone whose mail ends up in a box of some kind, it's important to know it's large enough to contain a rush of mail.
Then there's the elevation of the letterbox off the ground. Too high and not enough people can reach it. Too low and postal workers risk back trouble every time they go on their rounds. The guidance covers a wide range to allow for different designs of door, recommending that the centre of a letterbox be at least 700mm and at most 1700m from the floor. Thankfully not many letterboxes go for the full 1.7m.
There's more. The flap needs to open easily, and self-close. None of the edges should be sharp. The possibility of water ingress should be strictly limited. No viewing windows are permitted, to ensure privacy from the street. The whole thing should be corrosion-proof (the official test includes a 240 hour salt spray trial). And in communal locations the inside of the box must be lockable (or secured by some other device with at least 200 possible combinations).
When I moved to Bow my letterbox totally failed the security test. Poor design meant that anyone could walk up from the street and dip their hand in, removing anything they fancied. The day the postman delivered my new passport was a particularly stressful one. Thankfully the management company eventually replaced the old boxes with a new EN 13724-compliant design, but this suffers from the additional problem that the internal box slopes down at an angle. Post me a C4 envelope and it bends down fine. But most solid packages can't be tipped over like that, thwarted by length rather than height, which is one reason I don't buy much from Amazon.
The biggest change on moving here, though, was that my letterbox was no longer within my property. Previously I could pick up mail off my doormat with ease, whereas in my current set-up the communal boxes are at the front of the building on a completely different floor. I no longer know if and when I have new mail, I have to venture out to check just in case. This also means remembering to take my front door key with me and getting dressed to at least minimum standards. I miss the carefree days of instant mail collection - very much one of the downsides of living in a flat.
Meanwhile EN 13724 continues to be letterbox guidance rather than letterbox law, and came years too late for most of Britain's housing stock to have been affected. Should you ever need to post letters, cards, leaflets, newsletters or whatever around your neighbourhood, the risk of trapped fingers, bent backs and damaged envelopes remains.