Asset Postprocessor - Texture Resize Automatically

I want to resize all textures in downloaded package from asset store when they are imported. Because their resolution is too high so they has to been resized it but I don't want change the size of source textures at external texture edit program such as Photoshop.

How can I scale down as 50% when they're imported in OnPostprocessTexture(Texture2D texture). Any example scripts?

Thanks.