If I make A gui that perfectly fits my screen will it resize its self for other sized monitors so It does not get cut off?
It depends on if you use Scale rather than Offset in the object Size
Scale, given the name, scales itself to the size of all screens, while Offset uses set pixel amounts and will be the same width on every screen (if it is two inches on your screen, it will be two inches on everyone elses)
A way you can tell is by using the Test Tab in Roblox Studio and using the emulator to test different sizes and types. I also recommend making custom small sizes such a s a 480x360 screen to test emulations. Sadly borderpixelsize is always that many pixels and can interfer if to large. Avoiding this is a bit tough as you will need to make 4 custom sized frames using the scaling and fitting thr border.