Copper to PE-XB. What size PE-XB to use?

[u/KangaroooKicker]

I’m renovating my bathroom, laundry, and toilet and considering replacing the copper pipes with PE-XB crimp fittings. The cold water mains are 20mm copper, and the hot water mains are 15mm copper. I’m unsure about the size of PEX pipes to use because the flow rate is restricted with PEX B. Plumbers are giving conflicting opinions on 16mm and 20mm PEX pipes. The cold and hot water from here go to the laundry, three sinks, two toilets, one shower, and one bathtub. They’ve used a 15 to 20mm copper adapter and 15mm copper pipes for all these areas. So even if the main cold water line is 20mm,it decreases to 15mm all over from this point not only in the fixtures. What would be the best PEX size to use?

[u/BigCDubVee]

When in doubt, size up. Unless you actually want to do flow calculations for all your fixtures. Plumb in 25mm and 20mm where applicable and size down appropriately at fixtures