Generally sizes can be determined through the model number. Somewhere in the middle or at the end two digits represent unit size. With your model number I would interpret it to indicate about a 2.5 ton unit.
If you have no idea what size a unit is get the RLA off of the compressor.
Then go to some new product data sheets and compare the RLA of a new unit.
For example. You have a unknown compressor with a RLA of 15.5 A. Go to Carrier, Trane, Lennox etc. new product data sheets.
Under electrical it will give you the RLA for the compressors of different size units. For example a 3 ton system has a compressor of 16.6 RLA. And a Two ton system has a RLA of 14.9 RLA. Then it is a pretty good chance to think that you have a 2.5 ton system.