Detectors like LIGO, VIRGO, LISA that use interferometry have arms of a fixed basic length. At the end of those arms there are test masses that are free to move, but the displacement of those test masses will be very small compared to the length of the arms.
These types of detectors will be sensitive to wavelengths similar to that arm length for reasons similar to how the length of an antenna for a electromagnetic system influences which part of the EM spectrum it can detect. If the arm is much shorter than the wavelength, then the variations caused by the wave passing through are too small to see. Instead of measuring something where you see peak-to-peak amplitude differences, you will just be sampling points of the wave that are much less than one wavelength apart and therefore having relatively small difference in amplitude. If the arm is much longer than the wavelength, you are under-sampling the wave and cannot resolve it.
In any case, the magnitude of the effect from gravitational wave on the test masses at the ends of the arms is very, very, very small by human-sized scales.
So I think you're mixing in your mind the amplitude of the effect (very small) with where on the wave you sample the signal (could vary depending on the wavelength and the size of your detector, but typically "large").