How to get a font-size in between normalsize and small?

Question

I have some text that I want to be in between \normalsize and \small. Any way to do that?

Answer 1

Taking the average of normal and small size:

\RequirePackage{fix-cm}
\documentclass[12pt]{article}

\makeatletter
\AtBeginDocument{%
  \begingroup
  \normalsize
  \let\tmp@n@s\f@size
  \let\tmp@n@b\f@baselineskip
  \small
  \let\tmp@s@s\f@size
  \let\tmp@s@b\f@baselineskip
  \xdef\semismall@size{\fpeval{(\tmp@n@s+\tmp@s@s)/2}}%
  \xdef\semismall@baselineskip{\fpeval{(\tmp@n@b+\tmp@s@b)/2}}%
  \endgroup
}
\newcommand{\semismall}{\fontsize{\semismall@size}{\semismall@baselineskip}\selectfont}

\usepackage{lipsum}

\begin{document}

Normal size {\semismall not so small} {\small and small}

\lipsum[1][1-2] (\fontname\font)

{\semismall\lipsum[1][1-2] (\fontname\font)\par}

{\small\lipsum[1][1-2] (\fontname\font)\par}

\end{document}

I can't see how your reader will acknowledge the difference between normal size and semismall or between semismall and small.

Here is the same with a fully scalable font (Cochineal)

At 10pt main size

Addendum

Let me comment the image you got

I repeat the first image above for a comparison.

In the comment, you say that all sizes are the same, but it's not true. First of all, apart from scale factor and background, the typeset result is the same. If you look at the space between the period and U in the second line, you will see that it's smaller than the corresponding space in the fourth line: in the fourth line the font size is smaller and interword spaces are therefore larger in order to justify the text. In the 10.475pt part, TeX found no way to split “vestibulum” so that a part can go in the first line of the paragraph. If you look at “felis”, you can see that it's rather visibly to the left in the fifth line than in the third one.

This confirms my opinion: nobody can really spot that you're using a font size between \normalsize and \small.

I'd add that \small is often used for quotations, but in this case the part is set with wider margins and separate from the main text by vertical space, like with

\begin{quotation}\small
text to be quoted
\end{quotation}

This tells that the text is not original, but doesn't hinder reading, because the size is not so different from the standard one.

The \semismall trick might be used in a real emergency for cramming a bit more text in a paragraph, but in this case the second parameter to \fontsize must be \f@baselineskip so as to cover our tracks.

Answer 2

\documentclass{article}

\usepackage{lmodern}
\newcommand\bitsmall{\fontsize{9.5}{11}\selectfont}

\begin{document}

Normal (10/12)  {\bitsmall Bit small (9.5/11)} {\small Small (9/11)}.

\end{document}

The first number is the font size (default pt units) the second the baselineskip which is only used if complete paragraphs are set in that size, not as here just fragments in a normalsize paragraph.

I use latin modern here, if using the original computer modern, add \RequirePackage{fix-cm} as first line to allow fonts at any size.