As for batch processing the images you can use Gimp at the command line batch process or download the Batch Image Manipulation plugin called Bimp. Note:I haven’t used it, but I have it installed incase I need it. You can download the installer if you’re on windows or mac. For us Linux users we have to manually install it. Once installed, click File->Batch Image manipulation. If I remember correctly you can select the images or the folder with all the images. Then you add the process you want. In your case the resize. you can resize by Pixel or by percentage. Since these are textures they will be square(Same height and width). Remember a base 2 works best for GPUs. 1K(1024), 2K(2048), 4K(4096) or smaller like 512 and 256.
