[scribus] We should be able to change fontsize in latex editor

[Meho R.]

Ah, sorry for mess in my previous email, my email client got wonky. 
Here's another try:

There are many options. E.g., you can use any of these (usually 
installed by default):

\usepackage{fourier}% uses Utopia font
\usepackage{mathpazo}% uses Palatino font
\usepackage{mathptmx}% uses Times font
\usepackage{kpfonts}% uses KP Serif font

\usepackage[charter]{mathdesign}% uses Charter font etc.

There are other fonts with math support. More infos here:
http://www.tug.dk/FontCatalogue/

As Stefan explained, \fontsize{x}{y}\selectfont commands (where "x" is 
font size, e.g., 40pt, and "y" is baselineskip, e.g., 50pt) take care of 
the size of the text. You'd probably want to use these commands locally, 
i.e., in a group, inside curly brackets, as I did in the example below.

An example:

1. Open Scribus and create a Render Frame > Right Click > Edit Source
2. Tick off "Use Preamble" (since we'll use our own)
3. Type in this example code:

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\documentclass{article}

\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}

\usepackage{amsmath}
\usepackage{amssymb}

\usepackage{xcolor}

\usepackage{fourier}% uses Utopia font for text and math

\begin{document}

{
\color{red}
\fontsize{40}{40}\selectfont
Big text
}

\bigskip

{
\color{blue}
\fontsize{30}{30}
\selectfont
Math:
}

{
\fontsize{20pt}{20pt}
\selectfont
\begin{align}
I &
=a\sqrt{2}\int_{0}^{2\pi}\sqrt{1+\cos(\phi)}\;\mathrm{d}\phi\\
\intertext{\text{integrand is symmetric to
\ensuremath{\phi=\pi}, therefore}} &
=2a\sqrt{2}\int_{0}^{\pi}\sqrt{1+\cos(\phi)}\;\mathrm{d}\phi
\end{align}
}

\end{document}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

P.S. Also, there is another option: if you're using xelatex (in which 
case LaTeX command in File > Preferences > External Tools > LaTeX should 
be changed to: xelatex --interaction nonstopmode), you can load mathspec 
package and use any font installed on your system. However, this might 
not work perfectly in all cases (depending on quality of font used), so 
you should take care. More infos in mathspec documentation:
http://ctan.ijs.si/tex-archive/help/Catalogue/entries/mathspec.html

Regards,

M.