✓ Accepted answer
Sorry for modifying your code so much. You see, "Properties" and "TweenService:Create" has to do with the problem. You don't define what object you're changing, you're just changing what the properties are if you are using TweenService.
For example, if you wanted to change the color of a part, it would just be
local Properties = {Color3.new(0,0,0)}
also when you use TweenService:Create, the first one is defining what the parent is. So for the same color example, it would be
TweenService:Create(workspace.Part, tweenInf, Properties)
I respect that you tried, so I whipped up this code for you. Once again, sorry for changing it so much. Just put this in a local script inside of the TextButton.
local TextButton = script.Parent
local TweenService = game:GetService("TweenService")
local TweenInfoA = TweenInfo.new(0.5,Enum.EasingStyle.Linear,Enum.EasingDirection.In, 0, false, 0)
local Properties = {TextSize = 100}
local Tween = TweenService:Create(TextButton,TweenInfoA,Properties)
TextButton.MouseEnter:Connect(function()
Tween:Play()
end)
TextButton.MouseLeave:Connect(function()
Tween:Cancel()
TextButton.TextSize = 50
end)