Example of Terrain:Resize()?

How do I use Terrain:Resize? Can I use TweenService to resize the terrain smoothly as well?
Please provide several examples, not just links to different posts.

I’m not aware of any Terrain:Resize feature?

Maybe you could try Terrain:FillRegion and tween a bool value (on whatever axis you want to tween it on.)
Example of :FillRegion

local region = Region3.new(Vector3.new(0,0,0), Vector3.new(3,3,3))
region = region:ExpandToGrid(4)
game.Workspace.Terrain:FillRegion(region, 4, Enum.Material.Grass)

If I do that, would it also be possible to remove(subtract) the filled terrain via script?

Yes, though the only way to do that (that I know of) is by making it Air.