The default for exporting to LaTeX from org-mode files is to have the article document class at size 11pt font \documentclass[11pt]{article}
.
How do I get the exported LaTeX file to be a specified font size, namely 12pt?
Problably the simplest is to add #+LATEX_CLASS_OPTIONS: [12pt]
in you file, as discussed in the doc.
The LATEX_CLASS_OPTIONS keyword or EXPORT_LATEX_CLASS_OPTIONS property can specify the options for the \documentclass macro. These options have to be provided, as expected by LaTeX, within square brackets.
You can also use the LATEX_HEADER and LATEX_HEADER_EXTRA2 keywords in order to add lines to the header. See the docstring of org-latex-classes for more information.
An example is shown below.
#+LATEX_CLASS: article #+LATEX_CLASS_OPTIONS: [a4paper] #+LATEX_HEADER: \usepackage{xyz} * Headline 1 some text
\documentclass[11pt]{article}
at the top and then\documentclass[12pt]{article}
further down which doesn't seem to do anything.