It is possible to display the score in many different sizes on the screen. These sizes are called zoom levels. They have no affect on how the score will be printed (they only affect what is shown on screen) however, they make a great difference to how Sibelius performs.
At low zoom levels Sibelius will begin to run very slowly and at the lowest level, navigating the score is so slow that it is almost impossible. The number of staves in the score also makes a difference. In scores for solo piano, you can use much lower zoom levels however, in scores for large orchestras the zoom level may need to be increased to its maximum to make navigating fast enough.
Unfortunately, JAWS also throws a spanner in the works. When the zoom level is high, JAWS cannot recognise the text being shown on the screen. This means that when you want to edit a text item, you must first lower the zoom level so that JAWS can read the text properly.
There are no ideal zoom levels for navigating or text editing however, practicle experience has revealed that 400% often allows for smooth navigation, and 65% allows JAWS to properly identify text in most situations.
There are two keystrokes which are set aside for quickly changing the zoom level to 400% and 65%, and two more for changing the zoom levels in small steps.
The two keystrokes which jump to specific zoom levels are provided so that you can quickly switch between the best zoom levels for navigating the score and editing text.
You should get into the habit of always pressing Ctrl+Alt+Equals when you first open a score. This will ensure that you will always be able to navigate a new score without too much grief. If you find that the navigation is a little slow, even after pressing Ctrl+Alt+Equals, then press Ctrl+Equals a few times to increase the zoom level to maximum.