How can I get all the columns in my financial statement to fit onto one page?

You can edit the space available for the columns in your financial statement under General Ledger/Reports/Financial Statement by:

setting the 'Print Orientation' to 'Landscape' in the Column width (Printout) box;
removing the selection of 'Debit/Credit' on each column;
changing the 'Font size' in the Column width (Printout) box.