Wow what a loaded Question HOW big should a boiler be?
Well for starters I would take the length and the height and the width of the room in question to get my cubic foot area
It would also be a wonderful idea to get a heat loss chart to figure out how many BTU’s your going to lose from windows and non insulated walls.
The average degree day temperature should also be taken into consideration so your not over sizing or under sizing the BEST heating is the cast Iron radiators
Most folks take the area of the room call a supply house (plumbing) and say giveme sumting that will be heatin this space as the stumblebums can not be concerned about book stuff that may confuse them
You can Contact Harold as he is the EXPERT we EXPERTS contact.
Look for his postings and his web site HE is the best of the best. Shalom