What is wrong with the line of script below?

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.