Are you sure, you don't mix up normal dataTable-Tag with scrollableDataTable-Tag? normal dataTable has no height-attribute, and sounds. if you are looking for this tag. Here you control the amount of displayed rows with the rows-attribute.
You need the height-attribute, because the datatable is scrollable - therfore the name: scrollableDataTable. See the livedemo of richfaces scrollableDataTable, this is a good example!
Here is the link: http://livedemo.exadel.com/richfaces-demo/richfaces/scrollableDataTable.jsf