Try any of these and see if they work for you. Tex Notation does not seem to render a picture on Moodle anymore so I included a picture of what you should get.
\tiny gives size 5pt $$\tiny{40}\mathbf{_{\ 92}^{238}U}$$
\scriptsize gives size 7pt $$\scriptsize{40}\mathbf{_{\ 92}^{238}U}$$
\footnotesize gives size 8pt $$\footnotesize {40}\mathbf{_{\ 92}^{238}U}$$
\small gives size 9pt $$\small{40}\mathbf{_{\ 92}^{238}U}$$
\normalsize gives size 10pt $$\normalsize {40}\mathbf{_{\ 92}^{238}U}$$
\large gives size 12pt $$\large{40}\mathbf{_{\ 92}^{238}U}$$
\Large gives size 14.4pt $$\Large {40}\mathbf{_{\ 92}^{238}U}$$
\LARGE gives size 17.28pt $$\LARGE{40}\mathbf{_{\ 92}^{238}U}$$
\huge gives size 20.74pt $$\huge {40}\mathbf{_{\ 92}^{238}U}$$
\Huge gives size 24.88pt $$\Huge {40}\mathbf{_{\ 92}^{238}U}$$
\HUGE gives size 24.88pt $$\HUGE {40}\mathbf{_{\ 92}^{238}U}$$
