the size is usually dictated by the font, at least that has been my experience.
for simple reasons, you can't pass all group footers to one page, unless all your groups end one page. You could suppress the detail section in such a way that nothing is displayed. DBlank has done this more than me, but is something like:
in your subreport, you suppress the output, then
in your main report you go to section expert and suppress the section if blank.
You are not suppressing the detail section directly, so the subreport will run, but since nothing is displayed, CR will not display the space used by the subreport, which will just show the footers.
HTH