While showing homes to potential buyers, a frequent question comes up that is understandable, but also sketchy to answer; "Is the size of this air conditioner the right size for this home"? My answer, "Well...ah...duh...sure it is." 
Nah.  Not really.  It is always my policy to defer to the professionals in field.  In this case, I reference Steve Medina, a local Austin inspector.
 
Steve indicated there are many variables and factors to heating and cooling buildings that only experts know.  But there is a short cut you can do yourself is to find the tonnage or BTUs of your outdoor condenser unit. The condenser is the big square box (sometimes round) outside  the home with the loud fan blowing every time the A/C kicks in. On the back of that unit near where the coolant/electrical lines enter and exit is a label often referred to as the "nameplate". This label has many confusing numbers and codes that mean something to someone, but no body I know. Steve said the label gives indications of model numbers or serial numbers. The tonnage number is usually listed in the model number.
 
The number to look for is the one that has units of twelves. For ex:
 
MODEL CTF-36-12TI
 
The magic number is 36. This is usually the shortest number and is divisible by 12. Sometimes this can be tricky even for professionals, especially on older models.  The number 36 represents 36000 BTUs (12k BTU per ton). In this case this unit provides 3 tons of cooling energy.
Now you know...are do you?