Using Aviary Phoenix - pixels to inches?

I'm starting a new doent from scratch in Aviary's Phoenix. I want it to be 10 inches by 10 inches. It's default is 625 x 625 (pixels I ume). So what value do I need to change that to so it comes up as 10 inches. Can't find any info in it's help section. So how many pixels are in an inch?