For maximum efficiency, it is vital that condensing unit air handler sizes match that of the air conditioning unit. Manufacturer's specifications are listed on each unit, usually on the identification label along with model number and other information. If you are still unsure, visit a local home improvement center or call customer service with the brand and model description.
Use the manufacturer's installation reference to match the model numbers of the components, then it's a very difficult and can't be answered in the alloted space.
In order to insure you have the correct condensing unit/air handler match, you must have the tonage rating from the manufacture. For example, if you have a 3 ton condensing unit, you must have a 3 ton air handler. Rememeber 12,000 BTU per ton.
Check the condensor and evaporator units for their manufacturers information data plate. There is a model number and serial number on the plate, along with a manufacturers name and sometimes a phone number. Contact the manufacturer(s) to find the sizes. The closer the match the better the performance.
The unit outside is the condensing unit, the primary air conditioner unit. The coil in the inside air handler needs to be compatible with the outside unit, size and efficiency.