There appears to be a way to change the default fonts and size of fonts for new
documents under
Tools | Options | OpenOffice.org Writer | Basic Fonts (Western)

However anything set here appears to be largely ignored even if "Current
document only" is not ticked.

Another way that is somewhat more successful is to create a default document and
then save it as a template
File | Templates | Save | My Templates and give the new template a name and
click OK.
Then make that template the default template
File | Templates | Organise | select the template under My Templates | Commands
| Set as default template

However that only works when you create a new file under File | New | Text Document

If you go to Windows Explorer (obviously only under Windows - I have no idea
what the equivalent of this is on Linux or Mac)
and go to File | New | OpenDocument Text to create a new document it will not
only ignore any settings under Tools | Options | etc but will also ignore any
default templates.