Ground Turning Radius
Some things to consider: 


Don't make it a potential weak point on the aircraft. This was/is a big problem with some of the Mooneys out there where the nose gear can be damaged if a very tight turn is made while towing:



  One other caution: Many Mooneys suffer damage to the nose gear trunion
  when towing turn limits are exceeded via power towing. Owners learn to
  watch the ramp rats carefully.



Make sure you can maneuver the aircraft around the desired airfields. If you are building an aircraft that is intended to operate out of small GA fields with tight taxiways and short runways, keeping turning ability tight is key. If you are designing an airliner, make sure you can get around major airports (taxi diagrams are readily available). You don't want turning radius to be a limiting factor when buying/selling an aircraft.
To caster or not to caster, there are plenty of aircraft out there with castering nose wheels steered with differential breaking only this might have an impact on radius but a benefit in simplicity of design that is for you to decide.