After entering the Mathtype formula editor, edit it publicly. If you only change the size of a single top and bottom marker, please continue here. If you change everything, look below. After editing the promotion, select the upper and lower subscripts that need to be resized.

If you want to be smaller, select the small tab, click "Size", and then click "Smaller". If you want to set it directly to a fixed scale size, you don't need to change it one by one like this. Click Size, and then click Limit.