How would I get the current size of my screen (in pixels)?

Using the PlayerMouse Object from a GetMouse method of a Player:

--LocalScript

local mouse = game.Players.LocalPlayer:GetMouse()
local screenSize = Vector2.new(mouse.ViewSizeX, mouse.ViewSizeY)

Although this won't be accurate if the user resizes their screen, so:

--LocalScript

local mouse = game.Players.LocalPlayer:GetMouse()
local screenSize = Vector2.new(mouse.ViewSizeX, mouse.ViewSizeY)

game:GetService("RunService").RenderStepped:connect(function()
	local newSize = Vector2.new(mouse.ViewSizeX, mouse.ViewSizeY)
	if newSize ~= screenSize then
		screenSize = newSize
	end
end)