Typically for youth shirts, the typical customer who might be ordering custom shirts from you does not expect the design size to change, so 8x8 is suitable for the entire run.
However, we are very familiar with the fairly large variance in sizing between a Youth XS... and a Youth L & Youth XL.
If you decide to modify the design size based on shirt size, our recommendation would be to actually group your Youth L and Youth XL shirts with your Adult size prints if they are mixed into the order at 11x11 or larger.
And to print your Youth XS through Youth M at 8x8 inches or possibly 9x9 inches.
