11x17 paper (most print shops offer a variety of colors and different sizes or you can order kraft paper like I used HERE
If you're printing these yourself you'll just need to make sure you printer is setup to print 11x17 (not all home printers are capable of this) and you should also adjust the settings for thicker paper if appropriate.
But honestly it might just be easier to outsource the whole project and send the file to your local print shop (or maybe run a few copies on the office printer when your boss is at lunch, I won't tell!). Commercial printers can handle 11x17 paper no problem and most shop will have a selection of different papers to chose from too. So you probably can just have them take care of everything and move on to worrying about your Thanksgiving meal planning instead.
Interested in having me customize placemats for you? I'd be happy to! Just tell me a little bit more about your idea: text, graphics, colors, etc and I can set up a listing for you to purchase on my Etsy shop. Or I also have a digital download for a variety pack with 8 different designs here.