Recommended Spiral Eye Needle sizes match to specific brands of wool/yarn
SE-13 T Colonial Persian Wool, Wool Paternayan, & DMC Tapestry Wool.
SE-20T Appleton Tapestry wool, Colonial Persian wool, Wool Paternayan, DMC Tapestry wool, Planet Earth Silk
SE-22T Planet Earth silk and Elizabeth Bradley Wool
SE-24T Up to 6 strands of embroidery floss, size 8 Perle Cotton
Obviously there are other brands and types of needles and wool for needlepoint. This is intended as a guide only. Because Spiral Eye Needles thread through a slot in the side of the eye, the actual eye can be much smaller than when you need to get the frayed end of the wool into a traditional eye. It is hard to compare them to each other for this task.