how can I fix this script with tweensize?

Your Code :

local MaxHealth = script.Parent.Parent.Parent.Parent.Parent.Character.Humanoid
local Health = script.Parent.Health
local Bar = script.Parent

Health.Changed:connect(function()
    Bar.Size = TweenSize(UDim2.new(Health.Value/MaxHealth.MaxHealth,0,1,0)) ----- problem here!
    if Health.Value > MaxHealth.MaxHealth then
        Health.Value = MaxHealth.MaxHealth
    elseif Health.Value < 1 then
        Health.Value = 0
    end
end)

Code fix :

local MaxHealth = script.Parent.Parent.Parent.Parent.Parent.Character.Humanoid
local Health = script.Parent.Health
local Bar = script.Parent

Health.Changed:connect(function()
    Bar:TweenSize(UDim2.new(Health.Value/MaxHealth, "Out", "Quad", 1, false, nil)
    if Health.Value > MaxHealth.MaxHealth then
        Health.Value = MaxHealth.MaxHealth
    elseif Health.Value < 1 then
        Health.Value = 0
    end
end)

I'll explain your line of TweenSize here :

UDim2 is a Data Type composed of two UDims, one for the X coordinate and one for the Y coordinate, used to position and size GUIs. Link for explain UDim2 : HERE

Outis the EasingDirection and EasingDirection Enum Sets the direction for tweening. You can use : In, Out or InOut. For this i recommend you to use : Out. Link for explain EasingDirection : HERE

Quad is the EasingStyle and EasingStyle Enum determines the way in which tweening will act. You can use : Linear, Sine, Back, Quad, Quart, Quint, Bounce or Elastic. For this i recommend you to use : Quad. Link for explain EasingStyle : HERE

1 is how it will take time (float time) for Change the Sizeof the frame or the position if you use TweenPosition. here is 1 for 1 second. Link for explain float time : didn't found one.

falseis the bool override. Link for explain override : didn't found one.

nil is the Function callback. Link for explain callback : HERE

For more information about TweenSize go : HERE

Sorry for my bad grammar ^-^