The usual rule for note-type is two sizes less than the text of the work:
thus, to Pica work, Long Primer; Small Pica, Bourgeois; Long Primer,
Brevier. Side-notes are usually smaller in proportion. When side-notes or
references drive down below the lines of the text to which they refer,
the expedient of cut-in notes must be resorted to. This is a difficult
part of a compositor’s business, and requires skill and patience to
adjust all parts, so that every line of note and text may have proper
and equal bearing. The reglet or lead between the lines of matter and
the side-note must be cut with as much nicety as possible to the length
of the text, as far as where the note is to run under; and, having
accurately adjusted, by means of the quotations and justifiers, the
situation of the first line of the note, such lead or reglet is added to
the text as will make it precisely correspond in depth with the lines of
note that stand on the side before turning: the remainder of the note is
then set in a long measure, to correspond in width with the text, reglet,
and side-note; and the page is made up with note, or the text begun again
after the note is finished. In Bibles with notes and annotations, in
law-books, and other works, it frequently happens that a page exhibits
several of these alternate frameworks of note and text, which, if done
well, display a workman’s skill to great advantage.