Not all bonsai plants need to be wired though.
Wiring of bonsai plant requires a lot of care and attention. If you tighten the branches too hard you run the risk of breaking them altogether.
It is of utmost importance to choose a proper wire for wiring purpose. You have the option of either choosing a copper wire or an aluminium one. While making your choice ensure that you test the malleability of the wire.
The best thing would be to use an aluminium wire anodized to look like copper wire. Aluminium bonsai wires are comparatively easy to use than copper wires, although copper wires have more holding power.
You could either choose 100gm, 400gm or even 1kg rolls. Depending on the thickness, the length of the wire in each roll would vary.
One more advantage of using aluminium wires is that they can be reused, but copper wires cannot be reused. You also need to keep in mind that certain tree species are reactive to certain metals. For example copper is found to be harmful for species like azalea while iron and steel are found to be harmful for Junipers and other deciduous species.
The size of wire to be used depends on the size of the branch to be wired. Usually you could choose a wire which has a thickness equal to 1/3rd of the diameter of the branch or trunk to be shaped.