Button in the scale properties to resize the boundaries of a clip or layer.
Can you explain further? Expanding a "boundary" is meaningless for a video/photo layer - there are no pixels outside the original data. If you're referring to changing the outer boundaries of a clip to "cut off" pixel information, masks already do this. Dimensions of generated media can be altered by editing the plane's properties.
I feel I'm misunderstanding what you hope for here, as, what I think you're asking for can already be accomplished by other means.