According to the instructions I linked to (and other posts I've seen on this site), it's 3/16" for V-drives and 1/4" for direct-drives. So, I used 3/16". Those instructions say to cut the packing to lengths of 4 1/8". It didn't say if the length is any different when using 3/16" instead 1/4". 4 1/8" seemed just right for me. I cut the first length to 4 1/4" just to check, but it was just a bit long.

I saw other people post that getting the old packing out was the hardest part. That's why I made the little tool from a coat hanger. So, my only advice would be to make something like that.