You need a mears heat loss calculator available at all good plumbers merchants. If you give room sizes and number of outside wall to your plumbers merchant they will probably work it out for you. Boiler output is calculated on Btu's of rads + 20%.
Try http://www.cityplumbing.co.uk/heatloss.html. This website enable you to enter room sizes, window areas and wall types etc. Then calculates btu's required to heat room. add 20% to get your boiler output. Hope this helps