Hi,
I assume you are still developing your code, and you want to free up some space while still using the Debug build configuration?
The project is a 'managed project' within the LPCXpresso IDE, which means that the makefiles will be generated for you. After building, you can find the generated makefiles within the Debug or the Release folders.
If not done already: select your project, then go to Project > Properties > C/C++ Build > Settings > Tool Settings > MCU C Compiler > Optimization. Check the selected build configuration above, and set the Optimization level for Debug to -Og.
Other gains can be made by linking the release build of the chip and the board library to your debug build of the application, or by selectively changing build optimizations for specific C files - not advised. Switching at project level to -Og is by far the easiest way, which gives quite a benefit while still allowing you to debug your code easily.
Kind regards,
Dries.