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.
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.
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.