The size of your Lennox Air Conditioner can be determined from the model number of the unit.  The model number (M/N) can be located outside on the panel along with the serial number.

For Example:

A unit with a model number of 10ACB42-1P lists the unit's capacity that it provides to the living space. 42/12 = 3.5 Tons of cooling.

A unit with a model number of HS10-261-1P lists the unit's capacity that it is capable of producing. 26,000 BTUs: 26/12 = 2.17 Tons in which .17 tons or 2000 BTUs are needed to overcome motor heat and other factors and 2 tons or 24,000 BTUs are available for the living space.

There are 12,000 BTUs (British Thermal Units) per ton of AC cooling. Unit size or capacity has always been indicated in the unit model number in some way. Newer units go by the 12,000 BTUs per ton rule when annotating the unit's size in the model number which indicates the BTUs the unit can provide to the living space. Older units would indicated the actual BTUs the unit could produce which would be above the BTUs the unit could supply after taking into account loss  from motor heat and other  factors.

In summary older units indicate gross BTU capacity and newer units indicate net BTU capacity. For a chart that outlines and defines the capacity contained in Lennox model numbers Click Here