Why is "wavelength" used as a unit for the length of an antenna?
Antennas are designed for operation at a specific frequency, and the physical dimensions of an antenna scale with the design frequency. In the case of a dipole antenna, things like the antenna gain and directivity depend on the length in wavelengths, known as the electrical length. So if you know the antenna is, for example, a 1/2 wave dipole, you know immediately what the gain, directivity, and radiation pattern look like. If the length was specified in meters, then you would have to also know the design frequency so you can calculate the electrical length in wavelengths in order to determine the antenna characteristics. Also, it is possible to use electrical lengthening techniques which make the antenna behave electrically like it's longer than its physical length, complicating things when you know only the physical length but not the electrical length.

I will note that it is conventional for some antenna types to specify the size in wavelengths, and for others in meters. For example, dipole antennas are very commonly specified in terms of their electrical length in wavelengths, while dish antennas are commonly specified by their diameter in meters.