To be more specific, What radius would enable a ring-shaped space station to stably spin around Deimos and provide its occupants with rotational gravity?
Any radius (obviously larger than Deimos) will do, it will only change the angular velocity required to achieve the desired acceleration.
The rotation of a solid ring around an object is never a stable configuration, so you're always going to need ACS to keep Deimos in the center of the ring.
For greater efficiency, possibly some sort of wheel-spoke system can be built (a ground-based ring anchored to Deimos, and an outer ring using carts or magnetic couplings holding the "spokes" that anchor the station ring).
For instance, the average radius of Deimos is 6200 m, but its maximum radius is about 7200 m. Supposing we want to build the station on the plane where this radius lies, we need a first 45 km (45239 m) perfectly circular ring around Deimos, touching the surface in the maximum radius point and connected to Deimos with vertical columns with an average length of 1000 m.
On this ring, a suitable number of carts run continuously, each carrying a (say) 50 m tether to an outer ring, the station, with an inner radius of 7250 m. The station ring is then 45553 m long.
As an approximate rule of thumb, the seconds taken for a single revolution to achieve peripheral acceleration equal to one Earth gravity is twice the square root of the radius in meters. A radius of 7250 gives more or less one rotation every three minutes, and the speed of the "carts" would be about 900 km/h.