First you will need to know the inner dimensions of your chimney. There is only one way to find this out. You, or someone who can do this safely, will need to climb on top of the roof and measure the inside of the chimney (be sure that there are no fires burning at this time so that there will be no heat or smoke to contend with). Use proper safety precautions and equipment while doing this task.
If your chimney is metal or prefabricated then you will need to remove the chimney cap so that you can measure across the inner diameter of the chimney. With a metal chimney, a poly chimney brush is recommended so that it will not scratch or damage the interior of the chimney.
If your chimney is a masonry chimney then you will need to measure the flue's inner dimensions across both sides. The brush that you purchase will need to be a minimum of as big as the chimney liner or it will not be able to reach both sides of chimney, resulting in a poor job cleaning. However, you will want to be careful not purchase too big of a chimney brush either as this will make cleaning your chimney a very difficult task. Typically, wire chimney brushes are recommended for masonry chimneys.