Chest and length are the two you can't ignore. Chest determines how snug or loose the tee feels across the shoulders and torso. Length dictates where the hem hits—on the hip, below it, or way past. Sleeve length and shoulder width matter, but they're secondary for most tees.

If you're between sizes, think about how the recipient likes to wear tees. Do they prefer a fitted look or a relaxed one? For a gift, err on the side of roomier. A slightly loose tee is more forgiving than a tight one.