SteamPlayAreaResizer.cs

using UnityEngine;
using System.Collections;
using Valve.VR;

public class SteamPlayAreaResizer
    : MonoBehaviour
{
    private bool resized = false;

    public void Update()
    {
        if (!SteamVR.active)
            return;

        if (resized)
            return;

        var error = EVRInitError.None;
        var pChaperone = OpenVR.GetGenericInterface(OpenVR.IVRChaperone_Version, ref error);
        if (pChaperone == System.IntPtr.Zero)
            return;

        if (error != EVRInitError.None)
            return;

        var chaperone = new CVRChaperone(pChaperone);

        var sizeX = 0.0f;
        var sizeY = transform.localScale.y;
        var sizeZ = 0.0f;

        chaperone.GetPlayAreaSize(ref sizeX, ref sizeZ);

        transform.localScale = new Vector3(sizeX, sizeY, sizeZ);

        resized = true;
    }
}