Texture2D.Resize and Shader

I would like to know how can I reduce the resolution of the grabbed screen texture using Texture2D.Resize and Texture2D.ReadPixels. This texture is used in a shader after being grabbed.

I am actually doing this :

 RenderTexture.Resize(Screen.width / factor, Screen.height / factor, TextureFormat.RGB24, false);
 RenderTexture.ReadPixels(new Rect(0, 0, Screen.width, Screen.height), 0, 0);
 RenderTexture.Apply();

I could use the RenderTexture.mipMapBias but I want to reduce the texture's resolution.

Thank a lot !