I am not sure why you are asking. I assume by Adobe, you are reffering to Photoshop? This is the only reasson that begining size would matter. In this case it is not only size, but resolution that is important.
If it is a Photoshop design I would start at a resolution of 300PPI at the size of the final imprint. Sometimes I will create the design bigger than the average imprint size if I am unsure how large it will be printed so there is room to enlarge the image without sacrificing quality.
This is also the reason I create in vector (Adobe Illustrator) whenever possible. It gives you so much more freedom to do anything with the art after it is done with no size constraints.
If it is a Photoshop design I would start at a resolution of 300PPI at the size of the final imprint. Sometimes I will create the design bigger than the average imprint size if I am unsure how large it will be printed so there is room to enlarge the image without sacrificing quality.
This is also the reason I create in vector (Adobe Illustrator) whenever possible. It gives you so much more freedom to do anything with the art after it is done with no size constraints.