T-Shirt
Hem
Name the hem, then ask for the change you want: longer, cropped to the waist, a shirt-tail dip at center front and back, or a longer back than front. The same request works in pattern terms or relative to hip and waist landmarks on the avatar.
Examples
How to prompt
Say hem or hemline, then the change: longer or shorter, at the waist, sides higher than center front and back, or back longer than front. Landmarks such as hip and waist are the shared names when you think in 3D. See Body Landmarks for the full map.