You don't specify what format your spreadsheet is, but I will assume MS Excel.
Unless you have special viewers the preview will be generated by passing your spreadsheet through LibreOffice and convering it to PDF. My guess is that LibreOffice is not respecting the page set up either on the import or on the PDF export. I can't offer a solution other than getting a copy of LibreOffice and testing out how it displays / exports your spreadsheet under the desktop software and "fiddling" with the settings to see if you can get it to play nicely.
Bob Johnson