(at least where watch straps are concerned)
In order to establish the correct size/fitting for your watch, simply measure the distance (in millimeters) between the case lugs as seen in the illustration above. The determined width will be the size of watch strap you should order.
*In the event that you find the width to be an uneven size, ie. 17mm, 19mm, 21mm etc. and only in the case that you wish to purchase a leather or rubber watch strap - TSS recommends that you order the next size larger. The small degree of compression required to fit a 20mm strap to a 19mm lug width is well within the capability of supple hides. (while opinions will vary on this TSS has not yet had a single return arising from compatibility/fit issues when supplying the next size up).
**RE. Spring bars - if you are buying an oversize strap (as described above) it should be noted that the spring bars TSS supply with your strap will be approx. 1mm longer than the lug width of your watch. In this case TSS recommends that (unless damaged) you use the original spring bars supplied with your watch. TSS will be happy to supply spring bars in 17mm/19mm/21mm sizes if you request this via email immediately after placing your order.
Watchstrap lengths vary depending on who made it. Rather unhelpfully different watchstrap manufacturers around the world have different ideas of what constitutes a 'regular' or 'standard' length. For example, 'regular' length straps from European manufacturers can vary by up to 18mm in length and as such you should pay particular attention to the dimensions quoted with each item description.
TSS are unable to provide 'suitable for' wrist sizes or 'wearable length' due to there being several variables that make accurately determining wearable length somewhat problematic. The size of your watch, your watches lug to lug distance and to a lesser degree case thickness all need to be taken into account when determining the correct length needed.