If you are working with an illustrator, I would advise to ask for roughs at screen resolution (72dpi) and then final images at 600dpi which you can use for print and screen. Where possible, ask the illustrator to draw your characters in their birthday suit so that you can ask for different garment variations later which would be drawn on a different layer.
The face should be drawn on a separate layer too so that you can have various expressions. The
FlameBurst Mirai was 600dpi at 5000px x 8000px and was good enough to make a 1/1 scale print without looking pixelated or blurry.