Printing First Class postage on an envelope

Hi all,

I searched the forums, but couldn't find an answer. How can I print on a regular size envelope (#10)? ShipStation gives me a formatted envelope with address, return address and postage - but it always wants to print as 4x6! It prints out looking awful - all centered in the middle of the envelope with a few inches of blank space on either side. I tried changing the settings, but it's either 4x6 or 8.5x11, and 8.5x11 doesn't work at all.

Any ideas? Thank you!