From another angle, the frequency (and therefore wavelength) are set by the oscillator frequency (and any multipliers or mixers etc.) that produce the transmitter carrier wave frequency.
For the little DIY one or two transistor FM transmitters, the oscillator runs on the transmit frequency.
For serious communications radios, the oscillator frequency may be set by a quartz crystal then that fed through doubler or tripler stages to get to the required transmit frequency.
Or the modulated signal may be produced at a fixed frequency such as 21.4MHz (to make it easier to filter) then that mixed with a a separately generated signal from a crystal or synthesised oscillator, offset by 21.4MHz from the final frequency.
Some radios use several different intermediate frequencies and mixing stages to achieve the final output frequency.
Examples -
This is a block diagram for a relatively simple crystal-controlled radio; the lower half is the transmitter.
It shows the approximate frequency at each stage; from a crystal around 12MHz, which is tripled then doubled twice for an output frequency around 144 - 146 MHz; 12x multiplication.
(eg. for 145.5 MHz, the crystal would be 12.125 MHz)
Page 21 in this file shows a block diagram of a single-band handheld FM radio.
It also shows how a typical frequency synthesiser works, on page 14.
See page 3 in this one for a block diagram of a fairly complex multi-band radio: