Purpose To report the in vivo characteristics of the long head of the biceps tendon (LHBT); to evaluate the relation of age, gender, height, weight, and body mass index to the length and sutured and tubularized diameter of the LHBT; and to determine the smallest possible tunnel diameter for a subpectoral biceps tenodesis (SPBT) that can accommodate most patients. Methods The study included 66 patients (33 men and 33 women) with an average age of 54 years (range, 29‐73 years) undergoing SPBT. After tenotomy, the length from the biceps musculotendinous junction to the released end was measured. The tendon was transected 3 cm proximal to the musculotendinous junction and sutured, and the diameter was measured. The depth of the reamed tunnel was recorded. Results The average tendon length was 84.0 mm, measuring 91.9 mm in men and 76.2 mm in women ( P < .001), and the average tendon diameter was 4.4 mm, varying slightly between men (4.5 mm) and women (4.3 mm) ( P < .001). Mean bone tunnel depth was 17.5 mm, with 19 mm in men and 16.1 mm in women ( P < .001). Patient height showed a significant relation to both tendon length and tendon diameter. Weight was not correlated with tendon diameter but did show a significant relation to tendon length. Conclusions We have characterized the in vivo length and diameter of the LHBT at the time of an SPBT. Our findings have shown that there was a statistically significant gender difference in tendon length and diameter, but the diameter of the sutured tendon, which was placed into the tunnel, averaged 4.4 mm and ranged from 3.5 to 5 mm for all ages, both genders, all heights, and all weights. This finding is clinically relevant in that a small tunnel measuring 5.5 mm or less is sufficient to perform an SPBT. Level of Evidence Level IV, case series, anatomic study.