Are you saying my suggestion does not work - or, are you saying it doesn't work the way you believe it should work??
dokuwiki_site will expand to fit (frame)
page_group (and
div.page). If instead of setting a minimum size for
page_group (or
div.page) you'll have a tiny page in the top of a large dokuwiki_site - not pretty.
div.page uses height:auto (which is the "
somehow the size is based on the content"), use
userstyle.css to change that to min-height using the example I've given.
Look at the box model for a better explanation.