That would depend on the fixture demand for example are you talking flushometer 3.5 GPF or drinking fountains 1/2 GPM
http://www.masterplumbers.com/plumbviews/1999/watersupply.asp
Just because one may know the volume the pressure available should also be taken into consideration
the average home water line size is 1/2 inch to fixtures
Depending on the distance the pipe runs 3/4 inch pipe should be supplied to the plumbing location with 1/2 inch lines branched off to individual fixtures. I have also seen incidents where to much solder could cause restrictions,and even break off inside the pipes and clog shutoff valves and plumbing fixtures.
A pressure valve is usually installed on water systems to prevent excess water pressure from destroying fixtures or other plumbing components. When the pressure exceeds the valve ratings the valve opens and releases water/pressure from the line.
It depends on the plumbing code in your hometown. But realistically, I have seen houses with a 1/2 inch waterfeed supply 3 families. The bigger the diameter of the pipe, the more volume will incur in the dwelling. More than likely better to run 3/4" to each bathroom group with the last tee to your toilet.
LunchAns 2 - A main water supply line
That would depend on PITCH more pitch more fixtures allowed
supply potable water
never heard of the fittings,now you can run 2 fixtures off that line,not fittings
All potable water systems should be flushed after repairs and also if left sitting for extenden periods. I'm not sure if this is in the code but its just common sense. opening a hose bib or a tub valve when the water is restored after a repir is a best practise in the plumbing trade. If you do not do this, debris can get loose and stop up the fixtures in the home, such as toilets and faucet areators, which will result in a call back; i.e. the plumber comes back at no charge. Yes he should flush the line to protect the fixtures. The chlorine that most cities and rural water companies use will disinfect the line. Some cities require a plumber to disinfect a line but trust me be careful what you ask for because it will be at the home owners expense.
1/8 th -3/4 in per ft
Refer to a plumbing code book. Or call a plumber.
It depends on the plumbing code in your area.