Letter and A4 are paper sizes, so the size of paper you are printing with is the file type that you should be using. For the type of paper, cardstock works well. Look for paper weighted at least 110 pound cardstock or 165 GSM art cardstock I think.
On printing, Make sure your Pepakura and printer settings are both set to the size of paper you are using. Print preview should show you exactly the way the printed sheets should look as well.
You might want to check this thread out:
http://www.therpf.com/f24/pepakura-alternatives-fiberglass-85487/
Cotton fabric (and a lot of other fabrics) can be used with resin to reinforce, you just have to make sure that the resin is soaking thoroughly through the material to give it strength and a good bond. I've also used cotton cheesecloth and it worked very well when layered.