Changing a single note's size in a chord

Individual note heads in a chord can be modified with the \tweak command inside a chord, by altering the font-size property.

Inside the chord (within the brackets < >), before the note to be altered, place the \tweak command, followed by font-size and define the proper size like #-2 (a tiny note head).

\version "2.24.0"

\relative c' {
  <\tweak font-size #+2 c e g c
   \tweak font-size #-2 e>1
   ^\markup { A tiny e }_\markup { A big c }
}