Proper filtration depends directly on the size of the aquarium, obviously. Idealy, if you use media simialr to fine gravel or ceramic beads, you will need 2 cubic inches of this kind of bio media per aquarium gallon. The filter you are building will need to be able to turn over anywhere from five to ten times the volume of your aquarium per hour. If you are having trouble converting cubic inches to Gallons, that is aproxomitly .114G. So, if you are building a wet/dry filter for a 10 gallon, you will need aproxamilty 1.14G of biomedia. The pump you attach to that filter will need to be able to push about 100G per hour.
10G Aquarium - 1.14G/20 cu. in. BioMedia and 100G per hour pump.
20G Aquarium - 2.28G/40 cu. in. BioMedia and 200G per hour pump.
30G Aquarium - 3.42G/60 cu. in. BioMedia and 300G per hour pump.
55G Aquarium - 6.27G/110 cu. in. BioMedia and 550G per hour pump.
60G Aquarium - 6.84G/120 cu. in. BioMedia and 600G per hour hump.
If you are going to bulid a wet/dry canister, use a lenght of PVC piping with two endcaps to make the canister. Put your biomedia in a cheese cloth or nylon woman's stocking (so it doesnt get out of the canister) and put it in there. Drill holes into the endcaps and seal off some plastic tubing to both ends. These will be your intake outtake. If you are still trying to figure out how much pvc is enough. Use the area formula for a cylinder. (Pi)(r)squared multiplied by hight. So, aproxamilty 6 and a half inches of 2'' diamater PVC Pipe is large enough to hold the appropriate amount of biomedia for a 10G aquarium. Cool eh? Got any other questions? PM me.