"For instance, let's say
shows up as the last word on a line. In some cases, it splits so that it looks like this in the document:
You can beat this by being cunning. Change your sentence so that S/N is not at the end of a line. Especially useful if changes to the page margins (perhaps if you convert the file for Word users to read) will keep S/N safely in the middle of the line.
This is not a good solution - there is no reason for the text to split over two lines... I just want it to stay together. Why would I want to change the font size?
Changing font size would look strange. You would get a better result by changing Character Spacing for the symbol or, probably better, the whole line.