Hello!

We recently released an update that includes a variety of the most common letter and envelope sizes in Letter Box.

The list of available letter and envelope sizes are as follows:
Letter
Paper Size: 8.5″ × 11″ (216 × 279 mm)
Envelope: #10 Envelope
Size: 4.125″ × 9.5″ (105 × 241 mm)
Fits: Letter paper tri-folded into thirds (≈ 3.67″ × 8.5″)

Legal
Paper Size: 8.5″ × 14″ (216 × 356 mm)
Envelope: Legal Envelope
Size: 9″ × 15″ (229 × 381 mm)
Fits: Legal paper folded in thirds or once lengthwise

Executive
Paper Size: 7.25″ × 10.5″ (184 × 267 mm)
Envelope: #9 Envelope
Size: 3.875″ × 8.875″ (98 × 225 mm)
Fits: Executive paper tri-folded

Statement (Half Letter)
Paper Size: 5.5″ × 8.5″ (140 × 216 mm)
Envelope: A6 Envelope
Size: 4.75″ × 6.5″ (121 × 165 mm)
Fits: Half-folded 8.5″ × 11″ or flat 5.5″ × 8.5″ card

Monarch
Paper Size: 7.25″ × 10.5″ (184 × 267 mm)
Envelope: Monarch Envelope
Size: 3.875″ × 7.5″ (98 × 191 mm)
Fits: Monarch paper tri-folded

A4
Paper Size: 8.27″ × 11.69″ (210 × 297 mm)
Envelope: C4 Envelope
Size: 9″ × 12.75″ (229 × 324 mm)
Fits: A4 paper unfolded

A5
Paper Size: 5.83″ × 8.27″ (148 × 210 mm)
Envelope: C5 Envelope
Size: 6.38″ × 9.02″ (162 × 229 mm)
Fits: A5 paper unfolded or A4 folded in half

A6
Paper Size: 4.13″ × 5.83″ (105 × 148 mm)
Envelope: C6 Envelope
Size: 4.49″ × 6.38″ (114 × 162 mm)
Fits: A6 card or A4 paper folded twice (into quarters)

Quarter Letter (A2 Card)
Paper Size: 4.25″ × 5.5″ (108 × 140 mm)
Envelope: A2 Envelope
Size: 4.375″ × 5.75″ (111 × 146 mm)
Fits: Quarter Letter sheet, common for greeting cards

B5
Paper Size: 6.93″ × 9.84″ (176 × 250 mm)
Envelope: B5 Envelope
Size: 7.17″ × 10.12″ (182 × 257 mm)
Fits: B5 paper flat

Tabloid
Paper Size: 11″ × 17″ (279 × 432 mm)
Envelope: 12″ × 18″ Envelope
Size: 12″ × 18″ (305 × 457 mm)
Fits: Tabloid paper flat