If your eBook is a reflowable eBook, the page size will automatically adjust to fit your screen. You can further adjust the text size using the "Change font size" option. This is located at the top right of the screen.
If your eBook is a fixed layout PDF eBook, you can adjust the text size using the "Change page size" option which is located at the top right of the screen.
It is not possible to adjust the text size on fixed layout ePub eBooks using the desktop app.
If your eBook is a reflowable eBook, you can change the text size using the slider at the bottom of the page or by clicking on the "T" icon in the menu below the slider. You may need to tap on the page to show this menu. This option is not available for fixed layout eBooks.
For all format eBooks, you can also adjust the text using your device's zoom functionality.