It helps to know the size of a shorebird in order to ID it. One of the ways to judge size is to look at the shorebird in relation to other birds near it, especially other shorebirds.
In the above photo there is a mixture of shorebirds and Sandwich Terns who have black bills with the yellow (mustard, get it) tips.
Start at the top of the photo and you will see two shorebirds who are large. They have long, grayish legs and bills that are longer than the length of the head. These are Willets, a fairly common, large shorebird who is about 15 inches long. Having the Sandwich Terns, who are 17 inches long, nearby, helps confirm that the Willets are a large shorebird.
It's helpful to get to know Willets because if you're trying to identify a shorebird and it's standing near a Willet, you can immediately get a sense of that shorebird's size ( large, medium, or small) compared to the Willet. We say a Willet can be a good "marker bird," one that helps you measure the size of neary birds. Get to know a few other shorebirds well that can also serve as your "marker bird."
In the middle of the photo are a large number of meduim-sized (compared to the Willet) shorebirds whose bills are about the length of their heads. These are Red Knots (who are only red in their breeding plumage, not this winter plumage).
In the front of the photo, you can then see two other shorebirds. The one on the right, with the rusty back and brown "U" chest mark, is slightly smaller than the Red Knots. It's a Ruddy Turnstone.
After looking at the Red Knots and Ruddy Turnstone, who are meduim-sized shorebirds, it becomes clear that the pale shorebird on the front left of the photo is quite a bit smaller. This bird is very white below with dark legs, a pale gray back and dark bill. It's a Sanderling.
Practice this judging of relative size the next time you're near shorebirds.