Check your chair for holes drilled around the opening of the seat. Measure the distance between the right edges of two side-by-side holes and the hole width. Based on your findings for the distance between and width of holes, determine the cane size as follows: 3/8 inch distance, superfine; 1/2 inch distance, fine fine; 5/8 inch distance, fine; 3/4 inch distance, narrow medium for a 3/16 inch hole or medium for a 1/4 inch hole; and 7/8 inch distance, common.
Select a high quality cane that is strong and flexible. The side facing up should be smooth and glossy. Any bumps or "eyes" in the rattan where a leaf once grew should be smooth and unbroken.
Buy pre-woven cane webbing if there's a groove around your chair's seat opening instead of holes. Select a seat and/or back whose weave matches the size and pattern of the existing cane on your chair. Measure and compare the distance from hole to hole in the webbing to determine size. If no cane remains or you plan to replace all existing webbing, choose any size that will fit your chair's seat and back openings.