There are quite a few ways to achieve this and without specific knowledge of what you require its hard to suggest exactly what is needed, but here is a starter

Create a movie clip and then create two instances of it one called "right" and one called "left"

Use the set property command to vary their sizes. For example

Set Property ("/right", Y Scale) = 25
Set Property ("/left", Y Scale) = 75

Flash scales in relation to the movie clip's registration point so depending on what you need to do you may need to reposition the movie clip and/or registration point.

Hope this helps,

Rich.