Fire flow requirements will normally dictate the sizing as bimr suggests but the size of the fire flow may be dictated by legislation or utility standards. This is what you may need to adhere to.
In some jurisdictions minimum size mains are mandated, they may say minimum of 100mm for residential areas with normal housing, but it might be a minimum of 150mm or 200mm in industrial areas. You will also need to know what fire flow must handled and this will vary depending upon the type of properties etc.
If you forget about the fireflow requirement than you make the water main as small as possible capable of delivering the peak flow rates whilst still delivering the minimum pressure at the property boundary as required by whatever standard you have to deal with.
Smaller mains have less trouble with with formation of disinfection by products and dirty water.
Regards
Ashtree
"Any water can be made potable if you filter it through enough money"
