I've tried that, and I've chosen the "manual" feed tray option as well. Nothing is working. It should give me specific sizes for envelopes. A7 is a very common greeting card size.
I've also made sure that the printer screen is set to the specific A7 size. I think it's something about me being on a Mac using Adobe InDesign… they're not communicating with each other.
in the Paper size drop down you should be able to choose Advanced, Other Size or manage customs size.
Here you can write in a custom size and name.
My printer preferences doesn't have an A7 envelope option in the drop-down menu through InDesign CS3 (or any other program for that matter). I'm on an iMac Intel Core 2 Duo running OS 10.5.8.