From The Anvil Letter Plate and Cover Aged Bronze large and small,

Large Letter plate 83962
Overall Size: 304mm x 96mm
Opening Size: 232mm x 56mm
Fixing Centres: 268mm
Letterplate Thickness: 7mm

Small Letter Plate 83961
Overall Size: 252mm x 96mm
Opening Size: 204mm x 48mm
Fixing Centres: 230mm
Letterplate Thickness: 7mm

Anvil Letter tidy also known as a letter plate cover
83964 (large)
Overall size 354mm x 130mm

83963 (small)
Overall Size 265mm x 130mm

reduces drafts and helps tidy up and improve appearance of inside view of the door, this will cover up screws bolts and the opening for the letter plate.