At a glance I'd think 50mm x 50mm is doable. It's not a huge price difference, but you might as well save a few dollars if it doesn't make everything else more difficult.
Regarding the numeric keypad, don't those keys send two-byte scan codes? That may be what's goofing things up.
Thinking more about the board orientation, I just realized that once you've made the "sandwich", you can rest the boards on their edges and it's pretty stable.
But if you did shrink to a 50mm x 50mm board, you could fit the adapter on top of the Discovery board without covering the buttons - double win!
Let me know if there's anything I can help with on the layout or electronics. I'm no expert either, but I've done a few boards now and they all worked.For the rj11 keyboard connector... As you can see, this board is completely devoid of any electronics. It's purely an adapter board. So far, it's also the most advanced electronics I've done.
For a switch, wouldn't this have to be a 4PDT switch to get it right?
For autodetection, I've gotten several suggestions that are over my head, so... I'm not sure I'm going to get that right.
I'm kind of leading towards just changing it to be telephone handset cord only, and maybe I can get some help on figuring out how to protect the adapter when the computer's 5V is connected to the adapter's ground.
If you used a switch for keyboard vs. telephone cable, I think you're right you'd need some kind of multi-pole switch. Maybe just keep it simple and use a couple of jumpers instead.
Making it only use telephone cords sounds like a fine solution to me. The auto-detection would be really cool, but I know how it can be to have people clamoring to add random extra features to your projects.
But if you want to try it, I got a pretty good auto-detection suggestion from the EEVblog forums - let me make some diagrams and post them in a bit.