To properly measure a tarantulas true, outstretched diagonal leg-span or "DLS" there can be no noticeable bends in it's knees. If there are any bends this can throw off the measurement often by a an extreme margin. A scrunched up leggy specimen can measure as little as half it's actual DLS size!
I personally measure everything we sell and here is roughly how I do it. Keep in mind this is usually done with the tarantula still in it's enclosure for both the safety of me and the tarantula I am trying to measure.
I had this particular specimen out for a photo shoot she was pretty content in this pose. I figured it would be a good opportunity to take this picture with the caliper. Although the knees are slightly bent in this photo we can see the caliper reads approximately 2.6 inches leg-span.
Don't be discouraged, it is difficult even for me to get all the bends out of the knees so I usually measure them a little under their actual, fully outstretched size. This gives me a little wiggle room for their ad and I can never recall a customer complaining about getting a slightly larger specimen!
The absolute best device I have found to measure DLS is a caliper. This analog caliper (pictured) has been an indispensable tool in my arsenal for about five years. Just about every tarantula on the website is measured with this exact tool.