In general of chemical reaction, the amount of final product depends on the conversion and yield of chemical reaction ( initial mole input to reaction of starting materials). so your case you want the certain size with high amount of sliver nano., my option you can make optimum molar ratio between silver nitrate and sodium Brohydrat taht can give you the size as you want and then increase the there amount to double or more with increasing of solvent gradually. However, don't forget to keep the pH as can as possible almost same.