BTW, this is my first instructable, so my apologies if it sucks.
The first thing I did was try to compare the three sizes of kids I have with a lego guy!
I put three chalk lines on a wall, separated by 1 foot each line. I put the boys front of the wall, one by one, and took the picture. Then I photoshopped them together and, using another Lego Guy picture, then compared them to the Lego Guy proportions. I used the wall lines to figure out the measurements of the final costume too.
now.. I used the same structures suggested by the other instructable for the head. And yup, they are mega cheap. They are called Cylinder Building Forms, and I REALLY made a big mistake by not buying the smaller diameter tube :( You can see by the picture that I bought the size 12 one, but I think they had a size 10 one.
I then used a Huggies diaper box, because it was the PERFECT size!!! I made it thinking I'd use it as a template later. It was an awesome idea because it made my life soooo much simpler later!
I then bought three LARGE boxes from Home Depot and cut them into identical bodies. Easy Peasy.