You can write your topic however you want, but you need to answer these questions:
What do you want to achieve? Keep it simple and clear!
I want to find someway to make a part cover all the screen but be based on screensize and not just making it really big (it has offset of CFrame.new(0,0,-5))
What is the issue? Include screenshots / videos if possible!
I was using ViewportSize.X and works but since im using a SurfaceGui its not aligned as the original one.
What solutions have you tried so far? Did you look for solutions on the Developer Hub?
I looked on devforum but only found about position and not size
So you will need to incorporate the screen size(loads of ways to do this, I would recommend just using the Camera properties. You can use ScreenPointToRay to get a ray going from the camera to the direction of all 4 corners. You can do some trig to find the angle between the center and one of the rays(only need to do this math once). Get the dot product from the Camera’s LookVector to the ray you chose and then throw that value in an arc cosine function. Once you have the angle, you can then do some more trig to calculate the distance away the part will be using the 5 you already provided, that angle, and solve for the length ray you chose(Highschool trig, just ask if you haven’t learned SOH CAH TOA). Once you have that, add each ray’s position to it’s direction times the length. That will give you 4 points. Once you have the 4 points, you can use a CFrame to scale and rotate the parts between them all(should be pretty simple, ask if you need help with this step). That’s a lot and I’m sorry if you aren’t too experienced in math, but I hope this can help.