OpenOffice.org is not MS Office. Adjust.
Select "Format" under Category window. Scroll down to "Increase Font" under
Functions window. Now, select any vacant keybinding under Shortcut keys window.
Click on "Modify" button, Enter. Same applies to "Reduce Font".
When you press keybinding it will increase/decrease by two. (10->12, 13->11,etc.)
You might want to assign "Font Name" as well.
The actual file with defined key shortcuts is here:
Note: "Font Size" and "Font Name" do the same thing (cursor appears in Font
window), which is an odd thing for the former one. Bug?