Choosing the proper sized heating and cooling system in the Phoenix valley climate is mandatory, while also unique due to the extremely hot summer temps and the low humidity.
Our main goal here in the desert climate is temperature drop and not humidity removal as it would be in the mid-west.
Our rule of thumb has always been: 400 square feet per ton of cooling. In other words, a 1600 sq.ft. home would need a 4 ton unit.
This calculation has worked very well for us for the last 23 years. Always be careful not to undersize your unit; remember a smaller unit will remove humidity but will struggle to drop temperature of your home.