What is the minimum size envelope that the US Postal Service requires?

31/2" x 5"What is the minimum size envelope that the US Postal Service requires?
Not "too small", but "big enough".



Think about it!What is the minimum size envelope that the US Postal Service requires?
Has to be big enough for the address to be clearly readable, and the postage to fit.



Of course to small, and it will get lost in their machines, hence postcards have the poorest rate of delivery success.