You can customize the size and position of the caption area by creating a text box that links to the FVLPlaybackCaptioning component. To do this, follow the steps below.

First you will need to add captions to your Flash video. See more information on adding captions and subtitles to a flash video file using Adobe Flash Professional CS5.5..

Step 1- Double-click TextArea

In Adobe Flash CS5.5 open the file you want to adjust and select Windows > Components > User Interface and double-click on TextArea to add it to the stage.

Step 2- Configure the text box and properties

Size and position the text box, making sure that there is sufficient height to display 2 lines of text.  In the properties, change the name to CaptionArea.

Step 3- Adjust the Component Parameters 

Select the FLVPlaybackCaption component and change the captionTargetName field to CaptionArea. Also uncheck the autoLayout field.

Step 4- Test your changes

Test your changes by selecting Control > Test Movie > Test.