Call the local building department and ask them if they have any specific rules for sizing conduit that supersedes the National Electric Code. If they have specific sizing charts, you must follow the local building department's specification codes.
Read the jacket of the cable you want to run through the conduit. The size of the cable will be stated on the jacket of the wire. The size of the wire will be stated after the lettering AWG then one or two numbers will follow; this is the gauge of the electrical wire.
Read the conduit size chart for the exact size of the conduit needed, depending on your wire gauge and amount of wires. The most commonly used wire gauge in residential construction is 14, 12, and 10-gauge wire and you will most commonly run three to four wires through the conduit. If you are using this common size of wire you can use 1/2-inch EMT conduit. The sizing of the conduit does not matter, regardless what type of conduit you use either PVC or metal conduit.