If you need to change the font size of a specific part of an equation, e.g., a subscript of a variable or parameter,
scalerel is the only package you need. It is very simple to use it: the command looks like
text is the text which is supposed to be resized, and
size is the font size you want. Let’s try it out.
You should play with different sizes to get what you want. Overall, it is a powerful, easy-to-use, and useful package.