SchzamTry a ruler/tape measure, a device, hard material or fabric, with equal units of distance marked along one or both edges, with associated helpful numerals of the Arabic kind.
RuprechtUnless your wrist is extremely small or large, these should work and fit just fine. You can also use the quarter trick to cut the ends by placing a quarter at the end, cutting around it and then burning it (carefully) to reinforce the end.
http://www.dalucastraps.com/content/watch_sizing_chart.pdf