I have a text box with two lines of text with different font size. The text is left-aligned, but due (I'm assuming) to how the letters' bounding boxes are defined the two lines actually do not align. How do I align both rows to the left, in a way that makes the printed characters align?
There is a setting in the Story panel (found under Window -> Text) called Optical margin adjustment which does exactly this: adjusts the precise location of characters that fall at the beginning or end of a line to make the margin look more even.
It moves quote marks and such characters slightly beyond the edge of the margin to make the actual letters line up better, and it should also move the larger n here a bit further out to align the lines more accurately.
The number that you can set in points when you enable optical margin adjustment tweaks the degree of adjustment to apply. I have to admit I’ve never quite understood precisely what the number relates to and what it means, but essentially, the higher the number, the more rigorously the notion of a straight line is imposed. So if you set it to, say, 30 pt, it will move quote marks completely out beyond the margin and line up the letters exactly; whereas setting it to 5 pt will only move the quote marks out a small amount, leaving the letters more aligned than without optical margin adjustment, but not entirely aligned.