\documentclass[a4paper,12pt]{article}
\usepackage{amsmath,amssymb}
\usepackage[hang,perpage,symbol*]{footmisc}
\usepackage{array}
\usepackage{enumitem}
\usepackage{scrextend}
\usepackage{url}
\title{}
\author{}
\date{}
\begin{document}
\maketitle
\begin{tabular}{>{\hskip -40pt$}c<{$\hskip -10pt}@{\ --\ \hskip 15pt }l}
test & test \\[10pt]
f(x) = O(g(x)) & $|f(x)| \leq A|g(x)|$ for some constant $A$, and all values $x>x_0$ for some $x_0$. \\[10pt]
\end{tabular}
\end{document}
I want the f(x)... to have tiny font size but \tiny does not work

>{...}specifications are really not something you should be doing. – Alan Munn Jan 17 '16 at 02:56\tiny(which is really a size for text fonts rather than maths). – cfr Jan 17 '16 at 03:06