This is a suggestion. HTML will not make consistent pages (height or width) for a PDF document. What I have done in the past was to use a transparent GIF file to create the illusion of a regular page. If you then make that image the background image for the entire file - you get regular sized pages. This is important if you then covert the merged HTML files into a PDF file as well as it is then easier to look at as an HTML or PDF file. The attached GIF file is a simple transparent GIF file.
I would also like to suggest that html-merge be expanded to allow someone to select the page size from a list. The height and width is then just multiplied by 72 to achieve the size used for the background image.
Last, but not least, is that this transparent image be set to level minus one(-1) so it sits behnd every other image used. This would make it so it does not interfere with other background images (although - being transparent should already make it not interfere but anyway).
So that is my suggestion.