If you lived alone, only using one plumbing fixture at a time – correct pipe size wouldn’t be a big issue. However when you are in the shower and someone flushes the toilet – it is a big deal. The basic rule is two fixtures on a 1/2″ pipe. You need min. 3/4″ incoming cold pipe for a one bath house. Just running 3/4″ to each fixture in the house won’t hurt, but there will be no real gain. To size a water distribution system, get a copy of your state code book. It will spell it out in terms of beginning pressure, the furthest fixture from the meter and the number of fixtures in the house. Each fixture is worth so many units and you are allowed so many units for each size pipe as you get further from the meter. Be aware that this isn’t really a DIY job – you definitely should have help from a licensed plumber to design and install your plumbing system.