A2 would probably be the best possible size, as it's not overwhelmingly big but allows for a good amount of space for all the messages and pics. 420mm x 594mm. If you factor in the PET border, that still leaves plenty of room for contributions. Ideally, we'll want picture submissions to be formatted by users to something like 200 pixels by 150, as that way we can stack them on top of each other without having to do too much resizing. Obviously any pics already submitted, we'd have to reformat, but that's pretty easy with modern imaging tools.
I think it would be nice if we could have a general GAF message in the middle of the card. Just something simple like "Get well soon, from everyone at NeoGAF"
Depending on how many submissions we get, it might be an idea to have picture submissions on one side of the card, then have just regular well wishes on the other side. If we print off two sheets of A2, one with the PET monitor and pictures, one with more general well wishes laid out in text, then put them back to back, that should create a pretty comprehensive card that allows everyone to contribute in some fashion, even those without any art skills.
Again, this all depends on how many people submit something. We may find that an A3 size format is just fine for what we have.