Wow, I actually remember how to do this... sort of...
Given an equivalent speed to our current moon of 1,020 m/s...
Speed x Time = 2 π R (where R is the radius from the center of the planet)
R = Speed x Time / 2 π
R = 1,020 m/s x 259,200 s / 2 π
R = 42,078,020 m = 42,078 km
Earth's radius is approximately 6,371 km (from center to surface)...
Hope that means something to someone... heh.