Can “Page Sizer” Google Docs add-on read my Google Docs?

Tl;Dr: Yes, Page Sizer can read your Google documents.

To review the permissions that the app requires to be installed, click on the Install button, then dialog asking for the Google account to be used will be shown, then the OAuth consent screen will be shown. This dialog list the permissions required and next to each of them there is a button for each of them that show a description of what that permission allows to the app to do.

Below is a screen shot of the OAuth consent screen shown to me few moments ago: