The ideal size depends on the performance required of the system. It depends on the size of the boiler and the building. If you have long pipe runs or many bends in the pipes, they need to be bigger than if they are short and straight. If you have a big boiler, say greater than 25kW the pipes attached to the boiler will need to be 28mm just to get the heat out of the boiler fast enough. The initial circulation pipes may need to be 22mm, and then to the radiators 15mm. I wouldn't recommend smaller than 15mm as the system will have problems when the system gets neglected as they do.
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