I need to create a outdoor 2 for 6 meters with high qualite image.

I need to create an outdoor 2 to 6 meters with high quality image, so i use photoshop, but I don't know the proportion i suppose to use. i have heard about the use of just 10% of size, but I don't know if this will affect the impression, or i suppose to use a specific configuration on the delivery to the print shop?