A5 Letter weighing up to 100g. The letter/postcard rate of postage applies to items no larger in size than C5, the size of an envelope containing a folded A4 sheet of paper. Most letters, postcards, bills, statements and greeting cards fall into this category. To qualify for this rate of postage, items must weigh no more than 100g and meet the following size requirements: Minimum: 140mm long, 90mm wide, 0.18mm deep (with a tolerance of 2mm) Maximum: 235mm long, 162mm wide, 5mm deep