This pattern is written in one size, but with notes about resizing. It fits both myself and my 7 year old as it is.

It has lots of front and back post treble crochet, and links to a tutorial to help out with the pattern.

I used Rico Designs Creative Cotton and really liked the result, but I also made one in a mystery yarn that was more like wool, and it turned out nice too, so any yarn that meets gauge (or slightly bigger if you have large wrists) will work just fine.