You can do either one,but the 8 inch produces more spooled up engery off the line for racing.The smaller one will help the engine rev faster due to the reduced mass.It must be correct for the balance of the engine also.
It all comes down to application and personal preference,will it hurt the motor no,will it be the best one for your application ,only you know that.I also remember reading that small one and light weight ones do not do as good of job dampning the harmonics as a heavy one.Were talking minimal amounts but enuff to be noticed in crank bearing wear .Those are the basic differences that I personaly know about ,there may be more ?