Wrist measurements are measured as the circumference of the wrist where the watch is normally worn. Wrist measure can be done easily with a measuring tape or a strip of paper. 

Take a flexible measurement tape, and wrap it around the wrist where the watch is normally worn. The measurement can be directly read from the tape. 

Take a paper strip about 1 inch wide and warp it around the wrist where the watch is normally worn. Make a mark on the paper where the paper overlaps. This mark can be used to compare against a standard ruler to determine the wrist circumference. 

The conversion table for wrist size from inches to centimeters is below:

Watch bands are typically attached between the watch case lugs. Measuring the width between the lugs where the band is attached can be done with a standard ruler. Since the exact width between the lugs is essential for proper band fitting, measuring the lug width with a vernier caliper is often preferred. The lug width may be also available from the watch manufacturers catalog for the particular model. 

Lug widths are measured in millimetres. Typical lug widths are 18mm, 20mm, 21mm, 22mm, 23mm and 24mm - even through there are watches with non standard lug widths.