Using the folowing formula, you can calculate what size paper
you need to start with in order to fold an envelope of a certain size.

a = hight of finished envelope
b = width of finished envelope
A = hight of paper
B = width of paper

For example:
If you want the finished envelope to be 3 inches by 5 inches (a = 3  and b = 5),
then

A = (a x 16) / 5 = (3 x 16) / 5 = 9.6 inches

B = ((a x 4) / 5 ) + b = ((3 x 4) / 5 ) + 5 = 7.4 inches