Hard to say without seeing your model, but go ahead and start with:
- Check if you have unmet cooling hours (in eplustbl.html for eg, search for "Time setpoint not met"), as this could be an indication that the autosizing is going badly. That could be because of the what you have selected for the Summer/Winter Design Day rules for the schedules (ScheduleRuleset) that you use
- Read the eplusout.err carefully and see if you get any Warning/Severe errors that might hint what the problem could be
- Check your interior loads (equipment, lights, etc)
- Check infiltration and ventilation loads
- Check you have the right design days for your weather location
- Check the window (U value and SHGC in particular).
- ... hopefully you found a reason.
