More than any other printmaking process, silkscreen is less dependent on paper characteristics because you can print on nearly any surface. For a print requiring many layers of ink, a surface sized paper will keep the ink on the surface and allow layer after layer to be printed. Lightly or non-sized papers will allow the ink to sink into the paper.
Texture should also be considered. A smoother paper (especially suitable for photographic images) will produce a sharper image capturing more detail whereas a more heavily textured sheet will become a more pronounced part of the final image.
Dimensional stability is important for registration - look for a paper that will not greatly stretch or contract with moisture during the printing process.