Last night I was editing printed chapters. I noticed that on some paragraphs, the font size changes from 11 to 12. I used the same exact paragraph style on all of them. All my paragraph styles are set to size 11. When place the cursor on each paragraph, most show as size 11. Then, every now and then, one is size 12, and the paragraph style shows a + sign. I did not add any local extra info or override anything. The paragraphs before and after are 11. I used the same style on all of them. When I apply a an alternate paragraph style, also set at 11, the text turns back to eleven. Then when I reapply the proper paragraph style, the paragraph goes back to 12. I examined the style, and it is indeed set for 11. I did not alter any of these paragraphs locally. What could be causing this? How do I address this? The project is a book of 103 chapters, each on a separate file. I only have one character style and it is for a drop cap at size 14, so that's not it.
First, make sure no character styles could be altering the size as you described. Character styles take precedence over paragraph styles.
Next, turn on the style override highlighter. This can show you if the problem is caused by something other than the style itself. If you see the problem spots highlighted, then you should be able to fix it by clearing all of the overrides.
To clear overrides, make sure nothing is selected, then click the paragraph style flyout menu and click "Clear overrides". That should revert any text in the document to the "true" version of the paragraph style. Unfortunately, that feature doesn't always work.
If that doesn't fix the problem, you can select all of the text, then alt+click the style name to clear all overrides within the selection. This might be a pain for the book you described, but it's not that bad if you used threaded text frames and can select large amounts of text at once.