Water pump and pipe size for maximum water intake?
To achieve the maximum flow keep your pump at ground level. With a 1/2" slightly pressurized intake it may help to put 3/4" out put to the water tank but much larger would probably be a waste of $.  Make sure to have water in the pump at all times the shaft seals are usually ceramic with no water they overheat and crack. This would be another reason to keep the pump at ground level since there is low water pressure it will be difficult for the pump to prime. If you do have pressure on the 2nd floor it may work ok there if the pump max head pressure cannot make it to the roof. If memory serves fresh water weighs .43 pounds per foot so you will need the pump to be able to push a minimum of 25 psi for any flow from ground level to the top of the tank. I have a firend that gets water from a spring he pumps the water ~65' up the hill to a large tank and he has right at 30 psi in his cabin.