T-Shirt
Neckline
Name the neckline, then ask for the measurement change you want. The same request works in pattern terms or relative to the neck landmarks on the avatar.
Examples
How to prompt
Say neckline or neck, then the change: wider at the sides, deeper at the front, a relative amount in centimeters, or a full circumference target. Landmarks such as neck front, neck back, and neck sides are the shared names when you think in 3D. See Body Landmarks for the full map.