The classic Zentangles are made on 3 1/2" x 3 1/2"  paper tiles.  They were originated by Maria Thomas and Rick Roberts the founders of the Zentangle movement.  They prefer using an Italian made paper, Tiepolo by Fabriano.  The paper has a velvety woven surface, not completly smooth, but with some noticable weave.  It is also acid-free.  You can purchase tiles through their web site at www.Zentangle.com or make your own of any paper of your choice.