Each page requires information about its size. The given values will affect the page in multiple ways, depending on its type.
The size is always given in pixel.
In case a single color or an iFrame is used as a background, the background size won't affect this part at all, and can be chosen freely.
For an image, the background size needs to equal the image's dimensions.
(Also see here)
Elements, such as buttons or images, are positioned on the page via their absolute position.
The background's dimensions (or, in case of an image, the forced section's dimensions) are used as a frame of reference and 'drawn' on the user's screen in their original ratio, shrunk or grown to fit on dimension of the screen, while being smaller or equal to the other dimension of the screen, resulting in potential a border, either above and beneath the 'drawn' frame, or left- and right of it. Elements can only be positioned within this frame, so choosing dimensions with dimensions close to the user's device's dimensions is beneficial.
In case the checkbox hasBorder is checked, the mustSee-area's size is used instead of the dimensions of the image.
Elements can also be positioned using relative values, ignoring the upper part entirely, and being positioned relative to the display's size.