Here are the standard envelope sizes used in Britain and the paper sizes that fit in them
C6 114 x 162mm which holds postcards or A4 sheets folded into quarters.
DL 110 x 220mm which holds 1/3 A4 leaflets or A4 sheets folded into 3.
C5 229 x 162mm which holds A5 leaflets or A4 sheets folded in half.
C4 324 x 229mm which holds A4 Letterheads of A3 sheets folded in half.
C3 457 x 324mm which holds A3 sheets or A2 sheets folded in half.
