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!