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Kepler radius and similar term Keplerian radius are terms I can find no definition for.
I speculate it might be the radius of a Keplerian orbit that would produce a given observation: for example, if apparent the Doppler shift of spectral lines of a disk (e.g., circumstellar disk), planet, or binary-star companion indicate radial velocities implying some orbital speed, the Kepler radius would be the radius suggested by Kepler's laws. Another possible use, for a disk in which other forces contribute to the orbital speeds (e.g., gas pressure, radiation pressure), would be a radius of the disk at which those other forces cancel and the orbital speed is that of a Keplerian orbit.
The phrase Kepler radius also arises with a totally unrelated meaning: in discussion of Kepler Telescope discoveries, e.g., Kepler radius distribution, which could be the distribution of extra-solar planet radii in samples of Kepler discoveries.