In the end it is pretty simple the as the money supply has to be

large enough such there is a ready supply of capital in economy for the free efficient optimal exchange of economic goods and services
restricted enough such that the currency is not devalued as what happens when the supply of money exceeds the current demand, this fuels Inflation - Wikipedia which is bad
large enough such that the currency is not appreciating as what happens when the supply of money cannot satisfy the current demand, this fuels Deflation - Wikipedia which is bad