In your DefaultEngine.ini, Add the following
[SystemSettings]
r.SceneRenderTargetResizeMethod=2
//Reason behind this:
static TAutoConsoleVariable CVarSceneTargetsResizingMethod(
TEXT("r.SceneRenderTargetResizeMethod"),
0,
TEXT("Control the scene render target resize method:\n")
TEXT("(This value is only used in game mode and on windowing platforms.)\n")
TEXT("0: Resize to match requested render size (Default) (Least memory use, can cause stalls when size changes e.g. ScreenPercentage)\n")
TEXT("1: Fixed to screen resolution.\n")
TEXT("2: Expands to encompass the largest requested render dimension. (Most memory use, least prone to allocation stalls.)"),
ECVF_RenderThreadSafe
);