as a rule of thumb, single radiators 600mm high nominally give out about 1kW of heat max per metre length (in practice usually less unless your boiler is running very hot)
And double finned radiators nominally about 1.8kw per metre length
So you can calculate the amount of heat your radiators are capable of processing, by measuring them, and get a boiler of about that output.
Heating engineers usually do this, and also calculate the nominal heat loss of your house by inputting its size and construction details, this is so they can avoid quoting you for a more powerful system than you need, and so offer the cheapest possible system.
However in exceptionally cold weather, and if you want to heat the house quickly from cold, especially if you often are away, I like to have extra power in hand. It is quite useful to have oversized radiators because they can give a more gentle heat at lower temperatures. Modern condensing boilers run at their most economical at about 60C which is not especially hot.
The amount of gas used will not be greater, because your TRVs will turn down the rad in each room when it reaches temperature, and the boiler will modulate itself down to a smaller flame as demand from the radiators reduces.
Houses with combi boilers often have two or three times the power needed to heat the house, so you only need to consider if you want oversized radiators (I do)
Old boilers use to run hotter, so more heat came off the radiators than with modern boilers.