You forget to tell which unit you mean, but I guess you mean inches. (Most users here from outside USA measure in metric units.)
Anyway units doesn't really matter. The answer is that it's up to you. It all depends on the media you are using, your personal style and which expression you want the final print to have. Your question doesn't reveal any details ...
If you wish to have art go off the pages, or print to the edge of the pages, then you need to add at least .125" to each side - or increase the art by .25" in both width and height. Realize that extra eighth-inch will be trimmed off each side during production.
Otherwise, if art is not to print to the edges.. whatever size you want.