Wires are manufactured to standard sizes. The reel of wire will be labelled with the type of wire and the SWG. As the SWG increases the diameter decreases.
When using a wire in an experiment make sure you measure the diameter of the wire. Don't just assume you know the correct SWG; it is very easy to get the wires mixed up!
You will need a micrometer screw gauge to measure the wire diameter to one hundredth of a millimetre. Take one reading and then move 90 degrees around the wire to take a second reading. This will tell you if the wire is circular or not. Take several more pairs of readings along the length of the wire. Finally, take an average of all your measurements.
The Imperial Standard Wire Gauge (S.W.G.) is used in the UK. In the USA a different set of values known as American Wire Gauge (A.W.G.) are used.
http://farside.ph.utexas.edu/teaching/302l/lectures/node68.html Electromagnetic Induction
http://ww2.slcc.edu/schools/hum_sci/physics/tutor/2220/em_induction/ electromagnetic induction
http://www.school-for-champions.com/science/electromagnetism.htm Electromagnetism, with a quiz.