F11 to open Styles and Formatting window. Click on 4th icon from left to get Page Styles. Right click on Default, select Modify, select Page Tab. Now change your Page size from the dropdown (if needed) and adjust the tab settings as you require. Now OK out.
Apache OpenOffice 4.1.2 on Xubuntu 15.10 (mostly 64 bit version) and infrequently on Win2K/XP