So basically it's 16 long X 8.5 deep X 10.5 long. I just wanted it wide enough to set 2-3 warmers comfortably on the top for displaying. And that is the finished size. If you predrill your holes then it will be easier to assemble.
So, here's a view from the top before he put the top on. Notice also the corner posts. This was just a piece of base board that was the size we wanted. It's just squared about an inch wide. You'll want something to nail into. Also notice the yellow wiring what connects all three plugs. The only thing I would have liked better would be a plug turned sideways. By the time I went to check on it for more pictures, he'd already made these right side up. I did have him turn one upside down though. When everything is wired together that's when you hook up the cord and plug to it. At the end you nail on the top and leave the bottom open. We also drilled out a little hole for the cord to come thru at. That way the box sat flat on the table and I could plug it up to a 6 plug power strip.
Here's the finished box at my craft fair. I think it turned out great! We positioned the outlets high enough so I could still display a large warmer under neath and have enough room to see the plug in too. My warmers on top are plugged into my power strip behind.