fireball.Size=Vector3.new(4.5.6)
did you define fireball as a variable before using it also use commas instead of dots like this Vector3.new(4, 5, 6)
first of all you have to define fireball...
fireball=Instance.new("Part", game.Workspace)
then you did the (4.5.6) part wrong it is actually
fireball.Size=Vector3.new(4,5,6)
full script here
fireball=Instance.new("Part", game.Workspace) fireball.Size=Vector3.new(4,5,6)
If it is wrong tell me via PM.