Vector3.ClampMagnitude

Description

Returns a copy of vector with its magnitude clamped to maxLength.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
    public Vector3 centerPt;
    public float radius;
    void Update() {
        Vector3 movement = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
        Vector3 newPos = transform.position + movement;
        Vector3 offset = newPos - centerPt;
        transform.position = centerPt + Vector3.ClampMagnitude(offset, radius);
    }
}