The place for this question is the Sibelius tech support forumhttp://www.sibelius.com/cgi-bin/helpcenter/chat/chat.pl?groupid=3
wgere you can attach your sib file and someone will quickly tell you what is wrong
and how to mend it.
It does seem that you've been messing with font sizes
without also adjusting spacing. You can import one of the standard House Styles to clear
the decks. As mentioned, the Time Signatures (huge) text style, which is designed to
extend beyond the staff lines, may be a better solution. But let's see the score, over at
the tech support forum.