Yeah unfortunally its currently only for full HD (1920 x 1080). If you don´t have full HD please comment what resolution you have.
I have 1080 screen :( Game running in small window and fullscreen button is not clickable. Tried in Safari and Opera :(
Did you used the Browser version?
yup
There should be a button for fullscreen. Please check for it again. If there isn‘t one download the file and play the game using the downloaded file.