Unless the boiler was running full blast on the coldest day and never shutting down to keep the house warm, it is oversized! Dana has explained it numerous times, and you can read them if you search...a good way to see how much heat you actually used is to take your fuel use and match it up with the degree-day history files for your zipcode. You should have access to your fuel bills, and you can download the degree-day data for the period. That will tell you how much energy you actually put into the house for that period. You'd want to do that for a full heating season, and if you have the data, you could do it for several to get an average, since not all winters are the same. A little margin wouldn't hurt, but you don't want much. If you use huge setbacks overnight, the extra margin will help recover faster, but once you achieve the setpoint, that extra margin costs money in both efficiency and life of the unit.