We often get asked what size tapestry needle is needed for a certain count of fabric or canvas.  Here are some general guidelines for choosing needle sizes. 

Tapestry Needle Size               Aida                      Linen                     Canvas

            18/20                                                                                     10/12 mesh

            20/22                            6 ct                                                   13/14 mesh

            22/24                            8 ct                                                   16/18 mesh

               24                          10/11/14 ct              18-26 ct

               26                             16/18 ct                27-30 ct

               28                                                          32-40 ct

You always need to consider though the type of fiber being stitched with -- if you are using a particularly fragile fiber, sometimes it is advisable to use a larger needle than normal in order to enlarge the hole, and minimize the drag on the fiber.