ᐅ Installing water supply pipes for a new bathroom in the attic

Hello, I am currently installing a new bathroom in the attic. The hot and cold water pipes (both 20x2.25mm) are already installed in the bathroom. My question now is whether I should continue with these or reduce them to 16x20mm. The toilet tank is about 1.5 m (5 feet) away, the sink about 3 m (10 feet), and the shower about 6 m (20 feet). By the way, the shower will be a rain shower.

Thank you in advance for your answers.