I know how to do this in Scratch 2.0, but I can't figure out how to do this in 3.0.

when green flag clicked
switch costume to [small dot]
set size to (500) %
switch costume to [background]

This might not be what you need.
If it isn't tell me what you mean

Apollo11Leo wrote:

when green flag clicked
switch costume to [small dot]
set size to (500) %
switch costume to [background]

This might not be what you need.
If it isn't tell me what you mean

No, Im talking about backdrops, not sprites.

A sprite that covers the whole screen and is at the back layer is basically a backdrop.
You can't enlarge backdrops in scratch now unless you go into the editor and physically enlarge the background image, so you need to use a sprite for that.