Whenever you want to choose to have a small-sized equation in math mode, I would suggest the following
\documentclass[11pt]{article}
\usepackage{amsmath, amsthm, amssymb}
\begin{document}
\[ 3+3=\text{\footnotesize{\(x^2-13 =\)} \tiny{\( y^3-26 \)} } \]
\end{document}
Just be careful with parenthesis and spacing!