Sibelius’s Timeline can be a useful navigational aid in your score. But if you have limited screen space, or just don’t find it all that useful, you can easily switch it off by clicking on the “X” on the Timeline window, or unchecking View > Panels > Timeline.
To prevent the Timeline from appearing on each and every score you open henceforth, first, be certain that it is not showing. You might also modify the window of your current score to a size and position on the screen with which you often work.
Then, go to File > Preferences > Files.
Under Window Sizes and Positions, click the radio buttons for Open to custom size and position under both “When starting a new score or opening a score for the first time” and “When re-opening an existing score” and then click the Set Custom Size and Position button.
The message “Custom sizes set” will appear. Click OK to exit the dialog.
You should find that any scores you open no longer automatically show the Timeline, and as a consequence of these settings, scores will open at the size and position of your current score window.