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I believe you are referring to the headings. The "Shrink" option is greyed out because the headings should occupy the whole horizontal space. You cannot use "Shrink" in headings.
You can use the "Text" tool and then adjust or format the font size on how you want it to be. After that, you can just right click the field and then "Shrink" it.
If you double click the text field, the text editor will come up.
Let us know if you need any help. Thank you.