How do you update the checkmark sizes in the Graded Answer review slides?

Unfortunately, those checkmarks can't be edited. 

You could create a Review layer with custom checkmarks. However, that only works if you change the Shuffle setting to None. (If the responses are shuffled, you wouldn't know where to place the checkmark, because the position of the correct answer would change.) 

Another option is to use a Review layer to provide feedback about which answer is correct. 

FYI: The program will automatically create a Review layer if you enter text in the Post-quiz review field in Form View. Otherwise, you can simply add a layer title "Review" (named exactly like that), and the program will show the layer during the Review Quiz process.