The following is a fair rule of thumb estimate and no substitute for the calculation of an experienced professional on site.
For a combi you can ignore the heating demand in 99% of cases and base the size on the hot water demand. 1 bathroom and any boiler over 20 kW will do the job, though for a nice shower you should go between 25 and 30; 2 bathrooms, regularly used simultaneously, would require 35 kW or more. The amount of bathrooms used at different times is irrelevant.
Other important issues here are the size of the incoming water all the way back to the street valve, and the size and length of the gas pipe.
For heating only: 1 single panel radiator of 600 mm height and 1 metre length with “fins” on it equals 1 kW.
Add up the length of all the radiators and count “double panel radiators twice, the total will give you the size of the boiler in kW.
Due to the influence of insulation, and the current building regs, there is no need to include the cylinder demand in the calculation.
