The truth is, even the smallest combi boiler will comfortably heat the vast majority of larger homes. The average UK home needs just 6-8kW of heat on a very cold day, much less the rest of the year. Very large homes might need 10-12kW, but again this is on a very cold day.
Combi boilers commonly show outputs of 24kW-42kW, but this can be very misleading. These larger outputs on combi boilers relate to hot water performance. For example, a 24kW combi will give around 9 litres per minute (LPM) of hot water. A 38-42kW boiler will give 14-6 LPM.
The true outputs for heating on combi boilers are 16kW and upwards, which will heat the majority of larger UK homes.