A watch lug is where a watch strap attaches to the watch case. The lug size is the internal width between the lugs of the watch.
You can get a ruler with a millimeter and measure the internal width of the watch lugs.
Alternatively, you can measure the width of your old watch strap where it attaches to the watch case. Sometimes the size can be indicated on the inner side of the strap. Once you know the size you can easily shop and filter by lug width on our website.