As you can see from the profile of the Single Drawer Sewing Stand pedestal, the legs will attach to the slightly narrow portion of the base. As there will be three legs, they will be aligned at 120 degree intervals around the base. In this case the sliding dovetails will be 3 1/2" long. I'm assuming that, if you are building along for the first time, that you are working from a set of plans -- they are readily available on the internet.
I double check the depth when a place the router on top of the jig, and make the final pass. Remember, you will make only one pass, at the final 1/2" depth, for each dovetail mortice. This is possible because you have removed the bulk of the waste with the spiral bit. The result will look like this: