The EN 13724:2002 “Postal services – Apertures of private letter boxes and letter plates – Requirements and test methods”, European standard for letterboxes specify that:

• an envelope of C4 (229 mm × 324 mm) size must be deliverable without bending or damage;
• the internal volume must able to hold at least a 40 mm high bundle of envelopes;
• the height of an aperture is 30–35 mm;
• the width of the aperture is either 230–280 mm (for C4 width) or 325–400 mm (for C4 height);
• the mounting height of the aperture should be between 0,7 and 1,7 m.