The most commonly used diameter for pex is 1/2". Whatever you use, it must have an O2 barrier. A typical pex loop run should never exceed about 200'. You need someone to determine the spacing and you'd need the room layout to determine the best routing for the amount of heat you may need. A general guideline is to have the beginning of the run towards the perimeter since that tends to cool off the most, and needs the most heat. It is critical that the slab have insulation and a water barrier...no insulation and/or wet earth (may not be that common in AZ, but still potentially an issue) just sucks that heat out into the earth rather then keeping it in your slab.