inside a house, copper pipes are often 15mm (the size of your finger) or 22mm (the size of a smallish banana.
22mm has twice the capacity of 15mm
Bath taps need good flow, since a bath holds about 100 litres, to fill up quickly, so are 3/4" taps preferably on a 22mm pipe.
Basin taps are 1/2" on a 15mm pipe.
However, if the pipe coming into your house is only 15mm, and there is no cold tank in the loft, the maximum amount of water that can flow out of one, more or all your taps added together at any one time will be limited by that 15mm pipe. It might be in the region of 11 litres per minute.
So a huge combi that can deliver 20 litres per minute is wasted, because that much water is not available.
11lpm is quite a good shower, but inadequate if two people are trying to run taps at the same time. Hence combi owners often moan about fluctuations.
Incoming water pipes in modern houses are usually run in blue plastic. 20mm plastic has the same capacity as 15mm copper, because the walls of the pipe are thicker. 25mm plastic is like 22mm copper, and is adequate. 32mm plastic is better still and will give superlative flow. The cost difference in pipe is trivial, but valves are more expensive in bigger sizes, and sneaky plumbers may fit small ones, which will constrict the flow, hoping you won't realise. It is much more difficult and expensive to dig it up and change it than to do it right first time.
Also copper pipes inside houses are often fitted with cheap ball valves that constrict the flow (and leak). Full-bore ones of better quality are more expensive. So you may have to insist and monitor your plumber.
Cheap
good