ROM, or RAM? When it builds, the last thing in the console window should report the sizes of the text, data, and bss sections like this:
| text | data | bss | dec | hex filename |
91664 25980 10056 127700 1f2d4 project.elf
'Finished building: project.siz'
You can also look at the .map file that is generated to find more specific information about what was included, what was optimized away, and how much memory is being used.