Old Navy one size 2-3 years he has been wearing since he was 6 months old.
I found this tutorial (which links to this pattern), asked the little guy to choose some fabric, and we got to cutting. We used Moda precuts for both sides, and each side used almost a whole fat quarter. We ironed the interfacing on to the striped fabric, which ended up looking better on the inside.
Unfortunately, it is too small. The whole thing sort of balances atop his head, and the brim is a little flouncier than I had expected.
I am going back to the drawing board. I found this tutorial that would make a larger hat with a similar shape. This is a similar around-the-head style with a larger brim, which could be nice. I also like this one with sides that snap up and down, and this one that is more like a cap. Here is a pattern for sale that looks cute (what a cute baby, too).