This post will help you calculate the optimal length for an antenna at a frequency of your choosing.

Antennae are usually referred to in relation to wavelength. Therefore, to find the optimal length of an antenna we need to find the wavelength of the desired frequency. The formula for wavelength is 300 divided by the frequency in MHz. The answer will be in meters.

To save yourself the effort of doing some math, I have provided a link for you to Calculate antenna length.