Vector3.ClampMagnitudeを使うといい
http://docs-jp.unity3d.com/Documentation/ScriptReference/Vector3.ClampMagnitude.html
Mathf.Clampも使える
http://docs-jp.unity3d.com/Documentation/ScriptReference/Mathf.Clamp.html
http://docs-jp.unity3d.com/Documentation/ScriptReference/Vector3.ClampMagnitude.html
http://docs-jp.unity3d.com/Documentation/ScriptReference/Mathf.Clamp.html