Wells are normally sized ...

Wells are normally sized based on their ability to accommodate the submersible pump since one will generally be required unless well is artesian. If you need a small 100mm diameter pump then your borehole should be finished at a minimum diameter of perhaps 120mm. If you are looking at higher flows where a 140mm diameter pump is expected to be required then the borehole should be finished at 150mm diameter… and so on… Geology / hydrogeology can also be a factor with sizing because these characteristics will help determine drilled diameters… ie you may require the borehole to be lined with screen and casing and surrounded with a formation stabiliser / filter pack and this would mean the drilled borehole will need to be at least 100mm larger diameter than the outside diameter of your screen which needs to be large enough ID to receive your expected pump. The main thing is don’t undersize it ! Good luck