More than any other printmaking process, silkscreen is less dependent on paper characteristics as you can print on nearly any surface. Where paper is concerned it comes down to personal preference. 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, during which up to hundreds of colors are laid down one at a time.