How to do cancel the brush size circle?

Hi there,

Thanks for reaching out. I agree with @Myra Ferguson 
You may also check out this community discussion to know more about it. Let us know how it goes.

Regards,

Ashutosh

It looks like you have "Use Precise Cursors" enabled in the Preferences. To turn it off, go to Edit > Preferences (PC) or Illustrator > Preferences (macOS), and go to the General tab. There you can deselect it.