How to get physical size of the display/window? (not resolution)

Using the default sizing and positioning type of UDim2, you can scale the gui so that it is in the right location. Each screen is a different size, but roblox has already done the work for you. All you have to understand is that 0 is 0 and 1 is the other end of the screen for both the width(X) and height(y).

UDim2.new(number xScale, int xOffset, number yScale, int yOffset)

tb = Instance.new("Textbox")
tb.Parent = game.StarterGui
tb.Position = UDim2.new(0.5, 0, .5, 0) -- Will put top left corner of textbox in the middle of the screen

If you wish to get the exact size in pixels...

You will need to get the mouse from the local player(I'm using a local script in this example)

local mouse = game.Players.LocalPlayer:GetMouse()

Then, you can obtain the the width using mouse.ViewSizeX and the height of the screen using mouse.ViewSizeY

while true do
    wait(.1)
    print(mouse.ViewSizeX .. ", " .. mouse.ViewSizeY)
end

I added in a loop so you can test this in studio by resizing the side bars while in play solo

(Whole Script)

mouse = game.Players.LocalPlayer:GetMouse()

while true do
    wait(.1)
    print(mouse.ViewSizeX .. ", " .. mouse.ViewSizeY)
end

Credits to Legojoker's reply here(give him an upvote if this helps!): https://scriptinghelpers.org/questions/23265/how-big-is-the-roblox-screen

*Always try look up other scripting helper questions before posting your own in case its already been answered

Hope this helps. Let me know if this still isn't what you were exactly looking for.