That is quite odd. One of two things is happening: the Code::Blocks devs did not set a minimum size and they let wxWidgets figure it out in which case its a wxWidgets bug, or they just didn't set the size as large as needed.
The size is set in one of two places, the XRC file that contains the layout for the window (which would make it hard to change on just the mac) or the in the source file that creates the window. You probably need to have a look in buildoptionsdlg.cpp or something close to that.