I need help with my tweens, I'm confused, and it's not working?

Pointing out some issues:

Let's begin with the call of the TweenSize method. This function belongs to the abstract GUIObject class, meaning it's only invokable from an Instance of said class; you're halfway there, though, you've attempted to derive it from a property.

The method alone requires a total of six arguments, with one mandatory: the UDim2 final—which we can construct in-line.

Let's see how this would've looked:

Icon:TweenSize(UDim2.new(0, 0, 0, 0))

Addressing the problem:

The TweenSize method of GUIObject is designed to do just that, "tween" it's size. Though, from the looks of it, you appear to be trying to smoothly adjust a property. To achieve this, we have to take a different approach with TweenService.

To construct a specialized Tween designed to adjust a property, we have to build a customized tween-object with the Create function.

This function requires three mandatory arguments, that of which are:

TweenService:Create(
	The object we're tweening, 
	The details of said tween, 
	The properties to adjust
)

To provide the tween details, we use a TweenInfo Object; click the hyperlink to learn which details are assignable, and what each argument represents.

Bringing it all together:

local TweenService = game:GetService("TweenService")

local TweenDetails = TweenInfo.new(1) --// Time-length


local Icon = script.Parent


local TweenEnter = TweenService:Create(Icon, TweenDetails, {SliceScale = 0.25})
local TweenLeave = TweenService:Create(Icon, TweenDetails, {SliceScale = 0.1})

Icon.MouseEnter:Connect(function()
	TweenEnter:Play()
end)

Icon.MouseLeave:Connect(function()
	TweenLeave:Play()
end)