The templates will give you the size, but be warned, they're wrong. I made them to the templates and had to enlarge them. The help pages have the pixel dimensions, but I don't know if they're right either.
I'd suggest going by the pixel size and add a bit extra on.

Be careful, binders are fun to make but can be frustrating, I am now starting to get the hang of it. make sure you are looking at the design view & not the products view, make sure you stretch the entire image outside the box and completely over to the edge of the design box if you want the design to wrap or go to the edge, if you don't do this then you see the dreaded yellow triangle saying the design will not be printed to the edge. After publishing your binder to the MP go back into the product page, click customize, then click the design view then click on any of the views below your binder, make sure you don't get the yellow triangle, if you do then you didn't stretch the design over enough. For making binders that are made front & back indivually, I use 3901x4039 for the front & back measurements, for the 1 and 1&1/2 inch binders at 300 ppi, haven't attempted a 2 inch one since the darn triangle started to appear about a month ago. But like i stated before its very important that the design extends to the edge of the design view box not just to the red lines. Hope that helped.. I may try making a few 2 inchers with the measurements that gothic left.. If you are just using a solid background color with the hex code# & a small image in the center, inside the red dots then the above is not as important since the design or image is not intended to wrap over the edge. hope this helps!! good luck!