Specifying size of italics, math subscripts, and more in SizeFeatures

Question

Combining the suggestions of David Carlisle and egreg to refer to the Libre Baskerville font without knowing its name here, I ran into a new problem when I increased the font size to 12 points with the \setmainfont and \setmathsfont commands. Some font sizes are adjusted and some aren't (shown below). Specifically, italics in the body font are still at 11 points, subscripts and superscripts in math are at 12 points instead of being made smaller, and the roman font in math appears to be 11-point Computer Modern. Also, the line spacing appears to be a little tight. Possible clues: \small has no effect in body text, and the = is properly reduced in math subscript.

I figure that I need to specify some of these variant font sizes through various parameters to SizeProperties. But how? Or, can you tell me where to find the appropriate documentation on those parameters? I've found a few pieces, but so far nothing that spells it all out in one place.

\documentclass{article}
\usepackage{amsmath}
\usepackage{mathspec}
\usepackage{librebaskerville}
\setmainfont{LibreBaskerville}[
  Extension=.otf,
  UprightFont    = -Regular,
  ItalicFont     = -Italic,
  BoldFont       = -Bold,
  BoldItalicFont = -BoldItalic,
  SmallCapsFont  = BaskervilleF-Regular.otf,
  SmallCapsFeatures={Letters=SmallCaps,Scale=1.2},
  SizeFeatures={Size=12}
]
\setmathsfont(Latin,Greek)[Numbers={Proportional},SizeFeatures={Size=12}]{LibreBaskerville-Italic.otf}
\setmathsfont(Digits)[Numbers={Proportional},SizeFeatures={Size=12}]{LibreBaskerville-Regular.otf}
\DeclareMathOperator{\sgn}{sgn}
\begin{document}
We might, indeed, at first suppose that the proposition $7 + 5 = 12$ is a
merely \textit{analytical} proposition, following (according to the principle of
contradiction) from the conception of a sum of {\small seven and five}.
[
S_j = \sgn \sum\limits_i w_{ij}S_i
]
[
w_{ij} = \frac{1}{p}\sum\limits_{m=1}^p x_i^m x_j^m
]
\end{document}

Answer 1

I believe you're misunderstanding what SizeFeatures is about.

If you want 12pt size, specify it in the document class options.

In the following I use .ttf fonts, because they're the only I have on my system. Note also that neither LibreBaskerville nor BaskervilleF have Greek letters, so I use the Baskerville version I have on my system. You may need to find some other font that adapts well.

Also \setmathrm has to be used for operators.

Finally, since LibreBaskerville is quite big at design size, I set \linespread to 1.1 in order not to get crowded lines.

\documentclass[12pt]{article}
\usepackage{amsmath}
\usepackage{mathspec}
\usepackage{librebaskerville}
\setmainfont{LibreBaskerville}[
  Extension=.ttf,
  UprightFont    = -Regular,
  ItalicFont     = -Italic,
  BoldFont       = *-Bold,
  BoldItalicFont = LibreBskvl-BoldItalic,
  SmallCapsFont  = BaskervilleF-Regular.otf,
  SmallCapsFeatures={Letters=SmallCaps,Scale=1.10791},
]
\setmathsfont(Latin){LibreBaskerville-Italic.ttf}
\setmathsfont(Greek)[Scale=1.30569]{Baskerville}
\setmathsfont(Digits){LibreBaskerville-Regular.ttf}
\setmathrm{LibreBaskerville-Regular.ttf}
\DeclareMathOperator{\sgn}{sgn}
\linespread{1.1}
\begin{document}
We might, indeed, at first suppose that the proposition $7 + 5 = 12$ is a
merely \emph{analytical} proposition, following (according to the principle of
contradiction) from the conception of a sum of seven and five.
[
S_j = \sgn \sum\limits_i w_{ij}S_i αβ
]
[
w_{ij} = \frac{1}{p}\sum\limits_{m=1}^p x_i^m x_j^m
]
\end{document}

Now also a comparison of sizes:

\documentclass{article}
\usepackage{fontspec}
\newfontfamily{\librebaskerville}{LibreBaskerville}[
  Extension=.ttf,
  UprightFont    = -Regular,
  ItalicFont     = -Italic,
  BoldFont       = *-Bold,
  BoldItalicFont = LibreBskvl-BoldItalic,
]
\newfontfamily{\baskervillef}{BaskervilleF}
\newfontfamily{\baskerville}{Baskerville}
\begin{document}
A%
{\librebaskerville A}%
{\baskervillef A}%
{\baskerville A}
\end{document}

The first is Latin Modern. As you see, LibreBaskerville is quite big and it probably would make sense to scale it down to match BaskervilleF, rather than the opposite (for small caps, remember).

\documentclass[12pt]{article}
\usepackage{amsmath}
\usepackage{mathspec}
\usepackage{librebaskerville}
\setmainfont{LibreBaskerville}[
  Scale=0.9026,
  Extension=.ttf,
  UprightFont    = -Regular,
  ItalicFont     = -Italic,
  BoldFont       = *-Bold,
  BoldItalicFont = LibreBskvl-BoldItalic,
  SmallCapsFont  = BaskervilleF-Regular.otf,
  SmallCapsFeatures={Letters=SmallCaps,Scale=1},
]
\setmathsfont(Latin)[Scale=0.9026]{LibreBaskerville-Italic.ttf}
\setmathsfont(Greek)[Scale=1.17852]{Baskerville}
\setmathsfont(Digits)[Scale=0.9026]{LibreBaskerville-Regular.ttf}
\setmathrm[Scale=0.9026]{LibreBaskerville-Regular.ttf}
\DeclareMathOperator{\sgn}{sgn}
\begin{document}
Baskerville\textsc{Baskerville}
We might, indeed, at first suppose that the proposition $7 + 5 = 12$ is a
merely \emph{analytical} proposition, following (according to the principle of
contradiction) from the conception of a sum of seven and five.
[
S_j = \sgn \sum\limits_i w_{ij}S_i αβ
]
[
w_{ij} = \frac{1}{p}\sum\limits_{m=1}^p x_i^m x_j^m
]
\end{document}