Water pressure and flow rate

I currently have a problem with water flow and hot water feed to upstairs bathroom when the Kitchen sink tap is open. I am doing a loft conversion and a plumber mentioned make sure your builder put 22mm pipe feeding up from the new Megaflow Cylinder. Hpowever my builder says puting 22mm pipe will not help to resolve my issue. he further said if I want he can put 22mm for hot water and 15mm for cold water. Whether all these make sense. What is the correct pipe size for hot and cold water.