Not knowing the size...

I had someone, who has one of those programs where they can scan a picture
and make a cross stitch graf out of it, the question is, how do I figure out
the size of the aida cloth, to achieve the size that I'd want? I don't
remember how many squares per inch to determine the size, any ways I think
that's how it's figured out. Thanks, Carol In WI P.S. We are getting more
that just a tropical heat wave here in Wisconsin.