Tool - Soil Filter Trommel
A half inch mesh is a good choice.
Cycle wheel rims
Need three of these. The outer two on the drum are what the wheels with bearings run on and the center one is for the drive belt.
Ideally use cycle wheels from old broken push bikes. Cut out all the spokes because you only want the rims.
The size of available wheels will determine your barrel size. I only had 25" bike wheels available so my barrel is a little smaller diameter than I would ideally like. 25" wheels only actually have an outside diameter of 22.75". Typically the outside width of the rim is about 1-1/8" and the bit where the brake shoes used to act on is about 1/2".
Wheels with bearings
The center hole is 5/16" to allow fitting onto 5/16" threaded rod studding.
Motor (and Speed Control)
The best bet is to find an old exercise treadmill in a yard sale and pull out the motor, electronics, and speed control panel. This is ideal because the motor's ability to run at a slow speed means that you can avoid having to add a pulley stage to reduce the speed.
You can use a grooved drive belt directly to the middle cycle wheel rim.
http://www.amazon.com/Dayco-5060885-Poly-Rib-Belt/dp/B000C0WKMY/ref=sr_1_16?ie=UTF8&qid=1435099542&sr=8-16&keywords=Grooved+Drive+Belt&pebp=1435099622847&perid=07P6S81NPH0M48N0C9W5 88.5" long by .82" wide with six ribs $20
The rough estimate of the required belt length is 78" so this 88.5" belt makes the motor about 5" higher than is ideal but that is ok.
If you don't want to build a motorized trommel, it is still worth building a manual sieve. Make a frame from 2x4s that has outside dimensions of 3'9" and a length of say 6'. Wrap the wire mesh over it so that it folds down the sides by 1.5" all the way round. Use lots of big headed roofing tacks to hold it nice and taught.