FWIW, pex (and a lot of copper) has a nominal ID. What is called 1/2" pex or copper has the same 5/8" OD, but because pex's tubing wall is much thicker, pex has less flow capacity than copper (smaller ID) for the same tubing size. 1/2" pex should provide all of the volume a single showerhead can use...it might start to get deficient if you wanted to run two at once, depending on the showerhead, the distance, and the number of fittings required. By feeding the dwelling with a big enough pipe, and if you then branch out intelligently, each fixture should be able to get adequate volume.
There's a known friction (resulting in pressure loss during flow) based on length, altitude changes, and fittings for both copper and pex tubing, so it's not all that hard to calculate both the volume and pressure available at the end. FWIW, when there's no flow, you should have the same pressure at all outlets (at the same elevation), but it will drop once you open the valve if there's not enough capacity for the demand. Water pressure will change a bit over 0.4#/foot of elevation change, which is why they pump stuff up into a tower so the pump doesn't have to run constantly in many cases, and still have pressure in the pipes.