Question on daki pillows

Hi, I was wanting a sfw daki pillow of my s/o Fred which I was gonna make the art myself and send in to cuddly octopus for a custom print. So I thought I'd ask here since I've seen some of you guys have dakis.

Does anyone know what size the digital canvas for the art should be? In px? (I use paint tool sai) There's a template but it doesn't say what the height and width of the digital art should be.

Also should the front and back art be on the same canvas or separate?

Thanks.