这里要用到一个摄像机函数Camera.ViewportToWorldPoint, Unity官方对其定义为：
Camera.ViewportToWorldPoint 视窗转世界位置
function ViewportToWorldPoint (position : Vector3) : Vector3

Description描述

Transforms position from viewport space into world space.

从视窗空间到世界空间的变换位置。

Viewport space is normalized and relative to the camera. The bottom-left of the camera is (0,0); the top-right is (1,1). The z position is in world units from the camera.

视口空间是归一化的并相对于相机的，相机的左下为（0,0）；右上是（1,1）；Z的位置是以世界单位衡量的到相机的距离。

Note that it transforms a x-y screen position, into a x-y-z position in 3D space.

请注意它是变换一个x-y屏幕位置到一个3D空间的x-y-z位置。

You provide the function with a vector where the x-y components of the vector are the screen coordinates and the z component is the distance of the resulting plane from the camera.

这个函数带有在屏幕坐标的x-y组件向量，并且z组件是在从相机产生的平面的距离。

看了官方文档，现在我们知道怎么获取屏幕的实际宽高，下面是示例代码：

    private float width;
    private float height;

    void SetBasicValues(){

        float leftBorder;
        float rightBorder;
        float topBorder;
        float downBorder;
        //the up right corner
        Vector3 cornerPos=Camera.main.ViewportToWorldPoint(new Vector3(1f,1f,Mathf.Abs(Camera.main.transform.position.z)));

        leftBorder=Camera.main.transform.position.x-(cornerPos.x-Camera.main.transform.position.x);
        rightBorder=cornerPos.x;
        topBorder=cornerPos.y;
        downBorder=Camera.main.transform.position.y-(cornerPos.y-Camera.main.transform.position.y);

        width=rightBorder-leftBorder;
        height=topBorder-downBorder;
    }

得到的width与height就是屏幕宽高，代码的含义上面已经说的很清楚了。