Radio waves propagate through free space at about 300,000,000 meters per second common known as the speed of light. From that the wavelength can be calculated by the formula
λ = 300/f
Where λ is the wavelength in meters and f = frequency in MHz.
When electricity flows through solids it travels slower than free space. Data sheets for coaxial cable will include a correction value, known as the “velocity factor” for calculating physical wave lengths of the cable. Calculate the length in free space for the wavelength and frequency of operation. Then multiply that length by the velocity factor to find the physical length of coax needed.
Typical velocity factors for coax are between .66 and .90 of the free space length. Like all things electrical, there will be tolerances and you should cut long, measure and trim. If you need several identical lengths of coax from the same reel, you will usually be trimming the first piece and making the rest the same length.
A generic formula for calculating a needed length of coax is
L = (S * W * Vf)/f
L = Length in meters or feet depending on value used for S
S = speed of light in free space, use 300 if you want L in meters, 984 if you want L in feet
W = number of wavelengths desired, 1 = 1 wavelength, .5 for half wavelength, etc.