Ok, so this is one more thing where the shadow world government has failed miserably: Paper weight (technically, grammage) and dimension units, or – even worse – paper size standards. There is the big divide between continental Europe and the British empire, including its overseas colonies, but then there is also a whole mess of local customs, regional deviations, and odd preferences.
It seems nobody can agree on what size paper should come in, and how to measure its dimensions. Luckily, here is a convenient table for everything, and here is the Wikipedia article on the same topic.
Most projects described on this website will be based on190g-300g DIN A4 paper, which corresponds to 53 -82 lb bond/ledger and 8.27″ x 11.69″. Using US letter format should be fine, but you should slightly scale the templates to fit on the page before printing. If you are unsure about the grammage, just use sturdy carton which is still flexible and thin enough to be easily cut and folded.
If you use a Craft Robo, you have probably already found the thickest paper you can still cut through, and that should work fine for the projects on this site. If you are unsure about what the Craft Robo can digest, I suggest that you do some quick experiments. Note that there is a huge difference between a sharp new blade and a blade that has cut through a couple of dozen sheets already, so try to use a fresh blade.