Fractions in Google Docs equations are too small to read:
How can I write normal-sized fraction?
Left side: one usage of equation builder. Increase font size.
Right side: use another instance of equation builder.
In this example, 30px on the left, 18px on the right. The formula writer tried to keep the box (container for formula) the same height as the rest of the line height; shrink to fit.
Select the whole equation box, and change font size normally