Preferences are settings made by a logged-in user that control that user's account and various aspects of the user's viewing and editing environment.
To access the preferences screen (having logged on), click "My preferences" at the top of the screen, or navigate to Special:Preferences.
Along the top of the preferences screen are a number of tabs, which you can click in order to access various groups of settings. Depending on your screen size, you might wish to adjust the "Thumbnail size" (under "Appearance/Files"); this affects the default size at which many images in articles will be displayed. Don't forget to click "Save" after making changes.
For detailed information about available preference settings, read Preferences Help at Meta.