A rough idea of measurements would be something like below:
Bottom box: 4′ wide x 3′ deep
Middle box: 3′ wide x 2′ deep
Top box: 2′ wide x 1′ deep
These measurements are close to what I used for the top and middle tiers, but I made my bottom box a little deeper just to utilize all of the wood from my two 8′ boards.
I used 2×12’s for the bottom box and 2×8’s for the middle and top boxes
I used 2×4’s for the cross bracing that the middle and top boxes rest on
I used pressure treated lumber from the local big box store, but you could use Cedar or another similar rot-resistant wood suitable for outdoor ground contact.