You can format an envelope in any way you want. Envelope Addresses are normally formatted with the Envelope Address style and return addresses are formatted with the Envelope Return style. You can modify the styles in your merge document with whatever fonts you have available.
See
http://www.gmayor.com/changing_envelope_layout.htm
Put the address merge fields in the address frame assembled how you wish them to display. Put the return address on the envelope where you want and apply manual formatting to those parts you want in a different font size.