If you can make a rough estimate of the COP of this refrig cycle for R134a, then it should be possible to get some idea of the refrig capacity. R134a is stated to be similar to R12 in refrigeration behavior.
For example, if COP is approx 2.5 for an air cooled condensor condensing at 100degF, evaporating at -30degF, then refrig capacity, peak would be 105 x 2.5 = 263W = 897Btu/hr = 0.075tons refrig capacity
While refrig capacity, normal, continous, assuming the compressor runs for say 30% of the time = 263 x 0.3 = 79W = 269Btu/hr = 0.022tons refrig capacity
COP would of course be better in winter and lower in hot weather. Net cooling capacity would also depend on insulation from ambient heat leak into the fridge.
1 ton of refrigeration capacity = 12000Btu/hr
COP = Net refrig effect (sat vap enthalpy - sat liquid enthalpy, Btu/lb)/ heat of compression, Btu/lb