Go to the Product Page

This example uses a simple 2 x AA battery holder, Adafruit part 3858.

It has two mounting screws that we would like to know the diameter, as well as the distance between and the locations from the edges.

Select and Save

Look for a straight-on photo — a top-down (bird’s-eye) or bottom view of the part. Avoid angled shots or images with anything blocking the component, like batteries. In this example, use the second image from the bottom.

Right-click the image and choose "Save Image As .."  to save the image to your computer. 

Crop the Image

You will want to isolate the component. I'm using MacOS's built-in Preview App. Make a selection around the edge of the battery case and press the crop button (or CMD + K).

Grab the Dimensions

The Technical Details section of the product page has the components dimensions.