I know that full-screen mode is not available on Amara
But I did a subtitling job for a friend who had a video on YouTube and in Amara, the video was too small.
I told him to use ctrl and + but he asked for the subtitle file (srt) and managed to add it to his online video in YouTube
So now you have the video with full-screen and subtitled.
How did he do this please?
If your friend added the .srt file to the YouTube video, then the YouTube video is now subtitled. So when you use full screen on the Amara page where you made the subtitles, the subtitles that you see are those your friend added to the YouTube original which is streamed in Amara, not your Amara ones.
One way to see that clearly is to set a color for subtitles in YouTube, as explained in the Change caption font and format YT tutorial. This way you'll be able to tell YouTube subtitles from Amara subtitles in the Amara player (I've chosen to view YT subtitles in red for that reason).
I hope this helps a bit.