Laying water pipes for a new bathroom in the attic

Hello, I am currently in the process of installing a bathroom in the attic. The hot water and cold water pipes (both MV 20x2.25mm) are already in the bathroom. Now my question is, do I continue with this or reduce to 16x20mm? The flush tank is about 1.5 m away, the washbasin about 3 m, and the shower about 6 m. By the way, the shower will be a rain shower.

Thank you very much in advance for the answers.