The fact that the existing boiler keeps the house warm has only a very small relation to whether it is too big or not! Just read some of the other (many) threads here in this section. The easiest thing is to replace it with a similar size, and 95%+ of the time, that's too big, and often WAY to big. Figuring out what is the right size takes some actual calculations, not a guess, not just counting up the radiators, or the square footage, and no two houses are built the same, so you have to take into consideration the windows, sizes, types, insulation, amounts, types, locations, air exchanges (how tight the building is), and your actual design-day temperature requirements (available for free with your zip code). Until you've had that figured, while yes, you can put in a boiler that will heat the house, that is by no means the one that will give you the best economy while doing it...bigger costs more both up front, and every time it runs.