Several ways.
- Manually resize your source by dragging. If this is just a one-off thing, this is probably the easiest way.
- Include a reference source in your scene, and size it differently. Have its visibility hidden until you want to show the zoomed version.
- Create another scene with the zoomed version, and switch to it when needed (you can even use your original scene as the source, if you have a full composition)
Lots of different options, really just depends on what specifically you're looking to be able to do.