I’m a bit unsure as to whether the width of each of the 3 slabs should be 17.5" or 18.5" for the Mattone Barile 47" x 56" Insulated Base.
According to the picture in step 3, the width of the wood molds used to create the slabs should be 17.5". But according to step 5, the Styrofoam has a width of 18.5" and is to be fitted inside inside the center mold, meaning the center slab should be 18.5" wide.
In addition, the TOP of the base measures to 56" x 47" in the picture on page 4. This leads me to believe that the width of each of the 3 slabs should be 18.5", so that the three together side-by-side measure to 55.5", which is very close to 56".
Any help is appreciated, thanks!

Nominal vs. actual wood measurements. Each 2x6 is actually a 1½ inch thick by 5½ inch wide piece of wood. When you frame them as in step 3, you are standing them on the 21 ½ inch wide board, subtracting 3 inches total from its width (1 ½ + 1 ½ = 3). So the interior measurement of the mold is indeed 18 ½ inches (21 ½ – 3 = 18 ½).

Farther down the line, the three molds are making three slab pieces that will join together at the long (47 inch once molded) edges. Your center mold has space on either side—that space will run front to back along the slab on the completed base. The two molds that take the 27x8 ½ inch foam will be the front and back of the completed base.

The tough part in visualizing this is that you have to remove in your mind both the 2x6 pieces that form the mold sides, and the space underneath on the OSB. If you jump ahead to Step 10, you see what they should look like when the molds are finished. Imagine them without the 2x6 and the OSB, and that’s your slab.

Another way to say it: take 9 inches away from the combined length of the molds left to right (21 ½ + 21 ½ + 21 ½ = 64 ½ ; then subtract 9 to get 55 ½. (Why “9”? Because each 2x6 adds 1 ½ inches to the length, and there are 6 of them (⨉ 1 ½ = 9).)

"But it’s supposed to be 56 inches long!” I can hear someone saying (not you!).

The extra half inch is accounted for by two ¼ inch mortar joints that you will apply down the line, when you have placed the three slabs on top of the concrete block wall you will build.

So, at that point you are correct. It’s all in the steps you take to get there. Rest assured that the dimensions in the earlier steps are correct. It’s all in being able to turn the images in your mind to see where you’re headed.

I’m hoping this is helpful, but if anything still seems unclear, this is a friendly place and we definitely follow the “there are no dumb questions rule” here. Please stop by often as you proceed with your build. We want you to succeed!