how do i reduce the size of the login icons in mojave?

Do you mean the user icons for each user account on your Mac? If so I don't think so. You can, however, get rid of them entirely and just have the user name field and password field appear at login.

Can you post a screenshot of what you're referring to? I'm not sure i understand what your issue is.

Does anyone have a recommendation on how to get this on Apple's radar? The response may sound simple enough, but there are accessibility challenges introduced for some users on multi-user systems with the scrolling list of giant icons, and remembering and keying their user IDs is out of the question.

Feedback - macOS - Apple