Seiko Prospex SPB121 Specifications - Lug Width & Strap Size
The Seiko SPB121 watch has a 20mm lug width and is compatible with 20mm watch straps.
The Seiko SPB121 case size is 39.5mm in diameter and 13.2mm thick.
The Seiko SPB121 has a lug to lug measurement of 46.4mm.