Mary, you can use a wire gauge tool to determine the size. I would like to explain how to use a wire gauge.
If you review this picture of a wire gauge tool, you will notice that the top of the slit between each number/size is straight on each side. This is where your wire is placed to determine its size. Simply fit the wire between the slits until the wire rubs against them: not too tight, but still snug.
You can measure round, half round, and square wire with this tool. For half round, make sure you measure across the widest point of the wire. For square wire, place the flat sides of the wire between the metal slits.