Whats the best way for a component to lay out its background at native resolution?

Say I have a component that has a full screen background and I want to constrain the UI to that aspect ratio but also display the component, at least initially at the native pixel resolution. Ive been experimenting and I always seem to get an initial screen that is either scaled up or down a little, it would be nice to have the initial view completely unscaled. Im not sure if the MainWindows header is included in the aspect ratio hence the resizing I see happening?