Men Pommel Mushroom Size

[Tjr28]

Hi, I just bought a used pommel mushroom (no handles) for my 7 yr old level 3 son. He noticed right away there was not as much room for his hands when attempting a circle. I measured the width actoss and it is 18.5” and then I read 24” is the common size.

Is there an advantage for younger kids to start off with a smaller mushroom? Or is this size going to mess him up? I have looked online and I cannot find any mushrooms smaller than 24”.
Thank you!!!