If you are using snippets (Settings | Appearance | CSS Snippets), place this in a file in your Vault/.obsidian/snippets/ folder. You could call it big-math.css
mjx-math {
font-size: 200% !important;
}
Reload Obsidian, enable big-math under Settings | Appearance | CSS Snippets, and you should be good to go. You can play with the number using either % (percentages), or values in px (pixels), cm (centimeters), in (inches), as you like.