What hard tubing size should I use?

This is mostly going to be up to your personal preferences and goals with the build. 12mm and ½” tubing are small, don’t distract as much from other elements of the build, and are somewhat easier to bend. The fittings for them can also be more compact in size. 16mm has been gaining popularity for its ability to fill the case, show off coolant running through it, and overall take center stage, but can’t really hit the same tight turn radii that the smaller sizes can. 14mm is a good in-between size for those who want a larger tube than 12mm or ½” but don’t quite want the bulk of 16mm.