What formula do I use to calculate Gas/Pack (cooing-heating) size requirements for a 1700 square foot masonry home in the southwest?

I wouldn't dare make a guess. To many unknown factors. How well is the house insulated, how many windows and what kind, does the house get the sun all day, is it shaded, any heat producing devices in the house? You need to get the correct size or you will have problems. To small won't keep the house cool enough, to large and it won't dehumidify the house and you'll feel damp. You may want to call on a expert to look at your house and let you know the correct size. There are some quick formulas but I don't recommend using them.