"He who marches out of step hears another drum." ~ Ken Kesey
With those large size 15x17 or higher I would think that you all don't do any kid shirts or ladies shirts unless you set up different screens for those. Our standard print size is 12 x 12, but we adjust for a larger size print 13 x 14 or a tad larger if we are doing all adult shirts, we only have a few custy that want really big prints and will pay to have smaller print for kid shirts.
Our standard size covers up to 13" x 18" and we print that all the time. Oversized is 15" x 21" and we print those often too. Like Mimosa, it's a lot of "as big as possible".