If you are asking about the diameter of the shank of the needle, it's the universal domestic sewing machine needle size, a 15x1. If you are asking about the shaft size, that depends on the material you want to sew. Shaft sizes go from 8 (very thin for fine material) to 19 (very thick for heavy material).
You should Google "The Featherweight Shop". I'm not affiliated with them, but I have 7 Featherweights ranging from years 1935 to 1964. They have an awesome site that's all about Featherweights. Not only can you buy accessories from them, there is a section called "Schoolhouse" that is very informative. I have gotten lost for hours in the articles and videos they have on their site.
Enjoy your machine!