[foo92691]

There is no upper limit to the wavelength of gravitational waves. However, the lowest frequency that we can currently detect is around 10-100 Hz. Diving the speed of light (3e8 m/s) by (10 Hz) gives 3e7 meters. 30,000 km. The upper end of our frequency range is currently around 8 kHz which corresponds to 37.5 km if I did the math right.

LISA will be sensitive to much lower frequencies (longer wavelengths). NanoGrav also searches for these lower frequency signals.

We expect most gw's to be of very low frequency. High frequency g.w.s require tremendous mass and acceleration to generate. They are only generated in the final moments of black hole collisions as far as we know.