I understand the most general principle behind LIGO, but I am no physicist so I have no idea how to calculate the amplitude of gravitational waves, much less the length of the lasers required to measure them. Naturally, the farther away the source of the disturbance, the larger the observatory needed. So what if someone wanted to detect a mosquito on some exoplanet within a thousand light years? How large would this machine have to be? And what other things could we detect with it if it were built?
Well it really depends on the background. Obviously a smaller sensor would be more likely to detect the motion, but a larger sensor would be needed to detect a greater variety of signals and weed out the noise.
In practice there are more prosaic reasons like cost and tooling that limit useful lasers to a roughly human scale.