People keep talking about px. Mac and Windows use different DPI thus a reasonable size on one will be too small on the other.

Don't set a font size. Set anything that needs to be set relative to that; H1 is 200% bigger etc.

You have no idea what monitors people are using, nor what dpi they have, nor what resolution they're using, nor what fonts are on their machine.

Everyone can scroll. Not everyone can hit ctrl +, or install extensions like readbility, or install client side CSS.