You can find the options that govern the size and appearance of the score paper in the View menu. The commands in the first section of the menu will automatically fit the paper to the workspace in various ways. To go back to 100% zoom, choose the Reset Zoom command.
You can also the use the Zoom In and Zoom Out commands to adjust the level of zoom. These commands are available in the toolbar for quick access.
To see the hidden items such as invisible rests and bar lines in the score, make sure the Show Hidden Items command is checked.