Spiders are measured as if they were stretched out. A diagonal measurement is frequently used, crossing the spider from the tip of the top leg to the tip of the bottom leg. Examples of how we measure can be seen below.
Please keep in mind that when your spiderling arrives, it may look much smaller at first because it will likely not stretch out until it feels comfortable.
If you are still concerned that your spiderling is smaller than expected, please take a picture of it on a coin. We have found that is a safe and easy way for us to measure spiderlings. Email the photo to firstname.lastname@example.org.