Under the page layout menu, click on system 1, from the dropdown menu choose "Resize Page (Page Reduction)". You'll see the current reduction being used. Enter a smaller number and make sure "Page 1 through the end of the piece" is selected. Press OK.
Check your score to see if it's too big, too small or perfect. Repeat the steps above if needed. Once you have the correct size, respace music and update layout if needed.
Another option is to resize the staff systems. This option is also under the page layout menu.