To delete something, right-click on it in the Scene panel and select Delete from the menu. It will no longer be part of your effect, but it will still be included in the project.
To completely remove something from the project, delete it from the Assets panel too.
To rename something:
How to resize something depends on the object you want to resize.
For rectangles and the canvas:
For all other objects:
You can also change the scale, rotation and position of objects in the Viewport directly, using the Manipulators.
Yes. You can change an object's properties so that, for example, it appears while the user is capturing the effect but does not appear in the final video or photo. To do this:
Both boxes can be checked at the same time.