No matter how many times I made icing cookies...I never be able to make just right portion of royal icing and get leftover. Every time! Once you made royal icing, you can keep it for about a month in good condition, but it dries fast and I like to use the fresh icing on cookies (it's easier to pipe), so I basically just dump it when I have leftover icing..or maybe keep it for a day or two if I have another project to work with.
But if you have some leftover icing, I recommend you to make some small pieces...like I made some small bares and present boxes for Christmas cookies.