Let's explore the movie properties available in Flash: Click Modify > Document to get the detailed window shown below.
Dimensions (Size) refers to the size of your stage, which will also
be the size of your published movie. The default size of the stage is 550 pixels
wide by 400 pixels high. To make the movie just big enough to hold the contents
of your stage with equal amounts of space around the contents on all sides,
click the Match: "Contents" button. The Match: "Printer" button resizes the
stage to the maximum print area available for your default printer.
When in the Properties Inspector, clicking on the Size button will open the Document Properties window.
Background Color is the color of your stage/movie background. The default is white.
Frame rate refers to how many frames per second your movie will play.
12 is the default frame rate, which will work properly in most cases. There
may be times you'll want to change that. For instance, some animations appear
smoother when the frame rate is set at 21 or 31 - both of which are optimal
frame rates for Flash. (A theatrical movie plays at 24 frames per second.)
Be aware that the higher your Frame Rate, the faster your movie will play. This means you will need more frames in your animations to extend their play in the timeline. For instance, a movie with a Frame Rate of 12 fps and 120 frames will play for 10 seconds; the same movie with a Frame Rate of 31 fps takes only 3.9 seconds to play. In order to extend (or slow down) the play of the second movie from 3 to 10 seconds, you would need to extend your animation across 310 frames instead of 120 frames.
Ruler Units Flash uses this setting to display the measurements of rulers, grid spacing and the dimensions of your objects.