I found a quarter laying around, so I thought I'd take a couple of photos for size comparisons. Here's the WSTL18P1 with a quarter on its left, a euro on its right, and the case it's been designed to fit in at the top. The lid and a CR2032 battery are also seen at the bottom.

With a 0.922 inches diameter, the WSTL18 is just a few mils smaller than a quarter (0.955 inches) and just narrowly larger than a euro (0.915 inches). The next photo gives a closer comparison between a quater (left), a euro (center), and the WSTL18 (right).