This is based on my experiences with modding: Intentional Sizes: 32px - 64px (Good for Chrome Effect) 128px - 256px (Minor Details. Example iron sight or an additive lens.) 256px - 512px (This is the main body texture of the model)
Some possible combinations: 552px - 552px 1024px - 256px
The textures' color mode should be Indexed, not RGB. I use Photoshop to convert the color mode fromImage -> Mode located at the top right corner of your screen when in PS.