It's a wxWidgets-bug.
I don't know at the moment whether a workaround exists or not.
Might depend on your wxWidgets version.
What you can try, is to backup your default.conf on Mac and edit it manually.
Inside the <editor> ... </editor> tags, there should be <FONT> section, that can be edited.
On newly created default.conf's it's most likely not there, but you can try to add it manually.
On my debian-system it looks like (default-settings after opening the font-choser and closing it):
<FONT>
<str>
<![CDATA[Monospace 10]]>
</str>
</FONT>
I don't know whether it's a valid font-description on Mac, but I think it might be a starting point.
(Obviously the 10 is the font-size in points.)
But as told at the beginning: please backup your default.conf (at least if you have made any important changes), because editing it manually can always lead to errors !
I found a way to change editor's fontsize on MAC OS 10.6.8 using Code::Blocks 10.05
Open the default.conf file at ../USER/Library/Application Support/codeblocks/ with Texteditor.
Search for the FONT tag. It should be a tag inside the EDITOR tag and should look like this:
<FONT><str><![CDATA[0;14;75;90;90;0;Courier;0]]></str></FONT>
The second argument is the font size. You should also change font to Courier. I found that other font styles won't display the right size.
Save and your done. Restart Code::Blocks.