Just a regular CD-audio disc playable in a regular CD player. I suppose if you don't have a CD burner (or just want to be old-school) you could make a cassette tape or whatever works for you. But most people will just burn a regular audio CD (not an MP3 CD).
Everything is up to the creator. Generally the maximum is set at the limit of however many you can fit on a regular 80-minute CD-R. Epic double albums will be accepted as well, (like any other). The minimum is, again, up to the creator, but one must assume the theoretical minimum is one track. Unless, of course some month we have a theme like "hyper-minimalism" and everybody just sends in blank CDs. Or better yet, just empty envelopes. Then again, the concept of "mixing" does imply at least two songs. The bottom line here is that unless your aim is to be boring or post-modern, put enough on there to make it interesting.
No, but a CD case is a logical addition, and bitchin' cover art is encouraged although not required. Also, most people seem to like a track listing. Other extras have been included before, but they are up to you and not typical.
Also not required, but much appreciated, is electronically sending in the track list to email@example.com so that it doesn't have to be re-typed to post in the archive. If you have cover art, you can also send that to save the trouble of scanning. Ideally it should be 300x300 at 72 ppi. Please don't post track lists in the forum before the deadline, though; most people don't want to know what other people did until they're finished with their own.
If you're feeling really nice, you could also throw in a buck or two to cover the cost of shipping and such. Totally optional--your participation is what's most valuable. This is not about money, it's about music.
Send it to:play.list project
Padded envelopes are good ideas. It seems like postage for a padded envelope with a regular cd and case runs about one dollar, but make sure you check to be sure.
Yes, absolutely and please do.
Still confused? Send questions or comments to firstname.lastname@example.org.