The Standard Montessori Tray is the one we grab most often to throw together an activity. Our daughter loves this tray to use as a prop. It becomes a house, a swimming pool, a bed and multitudes of other things as a result of incorporating it into play.
1 3/8" deep
9 3/16" wide
If you are only considering a single tray, this is the one. It is a great size that makes it super-versatile and practical for instituting Montessori in your home.