If the title is not self explainitory… your ViewSizeX and ViewSizeY affects how you see SurfaceGuis. If you have a ViewSizeX of like 500 and your insert a SurfaceGui somewhere with a size X of like 700, anything outside of 500 will not show.
This bug should not really affect large computers (Ex: 1920x1080) but might affect mobile and low res computers (Ex: My 1336x960 netbook.)