Used the size chart on Merells own website and measured my childs foot to size. I cannot recommend these enough. I liked the quality. Water temperature should be warm (put room temperature filtered water in to top container, then fill to line with hot water, add salt pod and wait to cool. I got 2 of these actually, in different colors. It?s not pzoz brand and looks nothing like the one showcased in photos. I tried the skull candy krusher 3 and although they have a better battery life, they were incredibly uncomfortable and lacked the fantastic bass From wheels to the top about 6 3" tall and width definitely as indicated so do not be surprised. You can use it out of the box depending on your area, but I recommend buying the YAKIMA - SKS Lock Cores Car Rack System Components. I chose to play it safe .