The answer is full. Milady states directly, in its facial-shape design guidance: a full beard helps to fill out the narrow jaw.
This appears in a broader section on designing facial hair to complement face shape — for an inverted triangular face (over-wide cheekbones, narrow jawline), the book's specific recommendation is a full beard, not a square, rounded, or closely trimmed one.
Milady Standard Barbering, 6th ed., Ch. 14 (Men's Haircutting and Styling), p. 388: "A full beard helps to fill out the narrow jaw."
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