Currently, the page size must be uniform. We may add a "preflight" check that emits a warning when the page size is not uniform. Variable page sizes present all kinds of issues when the page size is fixed in an HTML container.
However, when we are able to support Liquid Layout, variable page sizes may not be a problem (though it will be irrelevent because each page will fill the screen).
Variable page sizes are currently supported in all non-slider formats (and Liquid Layout is also supported).