The eio file contains the autosized results. If you are running EnergyPlus by itself, it should be named eplusout.eio. If you are running it with EP-Launch, it should be named <modelname>.eio. And if you are running from OpenStudio, you'll have to check the run results folders to find the eio in there.
An example of this output file is below, which shows a header followed by the autosized flow rates, loop volume, and capacities.
! <Component Sizing Information>, Component Type, Component Name, Input Field Description, Value
Component Sizing Information, PlantLoop, COOLER_LOOP, Maximum Loop Flow Rate [m3/s], 1.60462E-002
Component Sizing Information, PlantLoop, COOLER_LOOP, Plant Loop Volume [m3], 18.05199
Component Sizing Information, Pump:VariableSpeed, COOLER_LOOP SUPPLY PUMP, Rated Flow Rate [m3/s], 1.60462E-002
Component Sizing Information, Pump:VariableSpeed, COOLER_LOOP SUPPLY PUMP, Rated Power Consumption [W], 457.15707
Component Sizing Information, Chiller:Electric:EIR, COOLERCHILLER, Design Size Reference Chilled Water Flow Rate [m3/s], 1.60462E-002
Component Sizing Information, Chiller:Electric:EIR, COOLERCHILLER, Design Size Reference Capacity [W], 343023.18468
If you are looking for the entire capacity for the building, you'll have to have a script read this file (it's CSV, so many options there), and add all the component sizing entries, then be a little smart about adding them all up so that you don't ever double count anything.
