I tried game.ServerScriptService.FireBallServer.Fireball.Size = 10.10
but it didnt work i am not sure why
That's because the property "Size" doesn't accepts at number as a value, it only accepts a Vector3
value.
Instead of giving it a number, instead create a Vector3
value and pass that as a value.
local FireballSize = Vector3.new(10.10,10.10,10.10) game.ServerScriptService.FireBallServer.Fireball.Size = FireballSize