Fresh water line system

[Heering80]

I found this water system from a local store: https://www.biltema.fi/rakentaminen/lvi ... 2000024223

It's reasonably priced vs uniquick or jg speedfit, so I'd like to give it a try. A straight connector 2,3€ per piece.
What do you guys think of the sizing? It uses 15mm lines. Is that too much or is it normal? I have understood many of you using 12mm water lines in your systems.

My setup is quite simple, just a kitchen tap + tap / shower in the toilet. But I'd like it to have a "normal" house water feel to it even if it means I have to carry more water with me. Does it require 15mm water lines?

Thanks!
Riku