Maybe this post can help:
https://forums.unrealengine.com/t/manipulate-horizontal-fov-without-affecting-vertical-fov/621443
But what I want would be an image filling the whole choosen window size, which is distorted depending on the vertical/horizontal field of view.
Do you want screen size to resize your image?
Anyway, I had problems with the Camera’s FoV, that how I set my Camera:
SceneCamera = CreateDefaultSubobject<UCameraComponent>(TEXT("SceneCamera"));
SceneCamera->bOverrideAspectRatioAxisConstraint = true;
SceneCamera->AspectRatioAxisConstraint = AspectRatio_MajorAxisFOV; // Important
SceneCamera->SetupAttachment(SceneSpringArm, USpringArmComponent::SocketName);