Manuscript Preparation, Copyediting, and Proofreading

Q. Does the font size of the footnotes need to be reduced in comparison to the main text font size?

A. In a manuscript, everything should be the same size. In a published document (whether typeset on paper or posted online), notes are usually set in smaller type.

[This answer relies on the 17th edition of CMOS (2017) unless otherwise noted.]