This is possible, as long as the scaling is the same for all your clips.
1. Scale your first clip. When you are satisfied with the result, select the scaled clip in the timeline and press CMD+C to copy the clip.
2. Group-select all the other clips you wish to scale in the timeline, then R-click on the selection and choose "Paste Attributes" from the list. The Paste Attributes window will open.
3. In the Paste Attributes window, select the "Basic Motion" checkbox (see pic) and press OK. All your selected clips will scale to the value of your first scaled clip.
Note: you can use this method to copy different attributes from one clip to one or more others, such as crop settings, drop shadows, video and audio filters etc...
Best wishes,
Ronny