Adapt size of layers

Not sure I understand, you can add any object to a layer that will be smaller than the base layer. The layer is just the canvas that shows the constraints of where objects can go, you dont have to fill them. 

Layers are just an overlay with the same maximum area as the base slide. The default background of a layer is transparent, so only the objects you show on them appear over your base slide. The feedback layers just have the small box with text added over a partially transparent background. You can change the settings of the individual layers to show, hide, and/or disable objects on the base or other layers, but the overall rectangle of the base will still show. You can change the layer background as you would the slide background, or add your own cutom rectangle as a background for the layer.

People often use layers to make floating menus, control bars, or other visual overlays, so the size is really whatever you want.