If the text and/or layout suddenly shrunk or grew then you might accidentally used the zoom out short-cut when you were viewing the page.
If you are using Firefox go to the page the sudden size-change happened and use this short-cut CTRL+0 or go to View | Zoom and click on reset.
Or if you are using Internet Explorer click on ALT to display the menu then click on View and go to Zoom and select 100%.
