The 3 standard size headphone jacks are - 2.5 mm, 3.5 mm, and 6.5 mm.
Pro audio tends to use the larger 1/4" (6.35mm+) with the most common being 1/8" (3.5mm) but some still use the 2.5 mm.
Time to get your tape measure out, vernier calipers work best.
