This excellent silicone rubber sports watch strap designed to fit watches with 20mm lugs is available in Red or Blue
Fitted with a stainless steel  side opening three fold clasp with a secure over-lock it is both secure, waterproof  and has anti allergic properties

Silicone Rubber straps are of course totally impervious to water and tend to stay more supple over their lifetime than do the resin and rubber types

At its longest the strap is 18.5 cms in length.  A small amount of length adjustment is  possible using the pin ladder adjuster in the clasp
Longer length reduction is easily achieved by cutting off sections of the strap  from the clasp end
Add the size of your watch case to compare to your wrist size.