How big is a yamika?

Quick question. I’m knitting my boyfriend a yamika for Christmas….or Hanukkah….late Hanukkah, lol. Anyhow, I don’t see them too often….don’t really know how big they are. I looked up ones on ebay and such but they seem to vary in size. Also, how….round? are they? Or is spherical a better word? Would you compare the shape to that of a small portion of a basketball? I’m having a LOT OF TROUBLE WORDING THIS!!!!!!

I’m mostly just looking for the diameter….think I can figure out the spherical-ness.