What's the best image size to use for making android icons?

I have Android kitkat4.4.4. I just downloaded an app called ICON CHANGER FREE so I can customize my icons. It takes an image from the photos saved on your phone and crop and it makes it an icon. My question is: what size image should I use with this app? And any particular file type?

I personally use 256x256 PNG format (If you want transparancy to be supported). 144x144 also works, but I always use 256 because it is easier to downscale an image as opposed to upscaling.