So I have a 24kW combi, with a 22mm supply pipe from the road (length 10 metres or so), until up close to the boiler. Pipes from the combi to points of use are mainly in 10mm PEX or 10mm copper. It is worth running large pipes to supply the combi with cold water, since that doesn't contribute to wasted hot water, and also other things such as the kitchen cold tap, toilets etc. will take water from that pipe, so if you have a large cold supply pipe, then the dynamic pressure drop at the combi input will be minimised when you're drawing off cold water to other things (e.g. if someone flushes the toilet, you don't want the DHW flow rate to suddenly drop). dynamic pressure ~ voltage at non-zero current (i.e. under the effects of "upstream" supply resistance). But for all of this, you need to know your static and dynamic mains pressures, demand sizes, and do some pressure loss calcs, otherwise you're shooting in the dark (and may make your life difficult + waste money by over-sizing stuff).