Pre-manufactered apparel for screen printing varies with the manufacturer and style of the item. Most manufacturers will have something similar to the below info. Not all manufactures have an adult and youth versions.
Adult: XS to 3X/4X
Youth: YSM to YXL
Adult: XS to 5X
Youth: YXS to YXL
Adult: XS to 4X
Youth: YS to YL
Sizes available for sublimated items:
4T > Extra Small > Small > Medium > Large
Extra Small > Medium > Large > Extra Large > 2 Extra Large > 3 Extra Large > 4 Extra Large
* Some items can be adjusted bigger if needed
*Custom sublimated apparel is close to industry standards for 4T to Adult 4XL.