#10 envelopes are 4.125" x 9.5".
The regular #10 is a dual window envelope has two clear cellophane panes. The top pane allows the return address to show and the bottom pane allows the recipient mailing address to show.
Letters with the address and picture on the first page are inserted into a #10 letter-sized, open faced, or full window, envelope. Your return address and the 3"x3" image both show through the window so the recipient immediately knows who sent the letter.