You can easily adjust the size of the subtitles by following the next steps:

1. First start the film/series you want to watch.

2. At the bottom right of the video screen there are some buttons. Click on the button that looks like a gear.  A menu will appear.

3. Then click the gear wheel next to “Subtitles” again.

4. In the menu, select the subtitle size of your choice under "Font Size".