We shoot for an 11" wide print on most designs, letting the length be dictated by aspect ratio. Of course the design itself may dictate other widths if it is long and thin for example. Our maximums are 12.5 x 15 for reasonable printing on the platens we have, and fitting it on a 13x19" transparency and 20x24" screens.
Since most of our orders have a wide range of sizes, we find the 11" fits most shirts well. It does get small on 3xl and 4xl shirts, but we don't typically have a large quantity of those in the orders. We also have orders where the customer specifically asks for a minimal design, say 6-9 inches wide.
I've printed up to 12" wide on a Youth XS before, many times. We almost never make multiple size screens for a single job. Customers won't usually pay for it either on small jobs. (It really becomes a separate job, so we would require a minimum of 12 garments for that screen.) And we never do multiple sizes for multi-colored jobs.
Someone asked about hoodies. We keep the length to a max of 9" and find we can print down to a Youth Large with that. With the smaller print field, the pocket balances the smaller print even on the larger shirts. If they have sizes smaller than YL, we cut the length even more and let them know it will be the same size on all the shirts they print. Alternatively, we've also printed larger designs on the backs of the smaller sizes that go on the front of the larger shirts when it's a front-only design. Customers tend to work within your physical press limitations.
