I ma trying to find out why one of my web pages scrolls really longer than I have set for.
No way of knowing without a URL. But web page height is determined by content, not explicit height values. Websites typically have both long and short pages. If your short page is especially light on content, it might be too small to fill your browser's window. Try adding more content.
I assume, there is a quite simple fix for this. But to nail it, we should have a look at the .muse file. Could you share a small sample file (by reducing the document to only one typical page) via Dropbox or a similar file sharing service with us?