These narrow, single wall postal boxes comply with the Royal Mail large letter size specification by all having a height, sometimes called ‘thickness’, measurement no larger than 25mm. The maximum weight is 750g and the length and width must be a maximum of 353mm and 250mm respectively. However, there is no need to meticulously measure your parcel when using our specifically designed letterbox friendly postal packaging; each product on this page will fit into the large letter category provided that the boxes are not overfilled causing the contents to bulge, enlarging the dimensions of the parcel.