Is there working firmware on the unit now?
from the command line will list the files and their sizes in flash memory. There may be some crash dumps and miscellaneous files that you can erase to free up space, or both the working and a backup firmware image in which case you can delete the backup.
If the flash still doesn't have enough space to hold both the current and new firmware, you can reasonably safely erase the current running firmware from flash.
During the boot process, the firmware image from flash is copied into RAM. Once booted, the router no longer references the firmware file stored in flash. Therefore you can erase it to free up space and then copy the new firmware to flash. Once you erase the running firmware, be careful not to power down or reboot the unit until your new file is uploaded and saved, and you have configured the unit to boot from it with the command:
boot system flash filename-of-new-firmware
If the router loses power or reboots during this process, all is not lost but recovery is more difficult.
If you are a GUI user, the procedure to erase the running image and copy the new firmware from a local file is fairly straightforward under the firmware tab. Be warned that during the copy process the web page will just sit there for a long time up to a few minutes while the file uploads. There is no progress bar or other indication that anything is happening. Don't refresh or leave the page until it completes. After the new file uploads, select it as primary, save and reboot.
OK, I missed the point that there is no firmware on it now. You will have to contact Adtran support as the 1200881L1 won't run any of the images listed on the website so they'll need to give you FTP access to an earlier image compatible with that hardware.
From the 3305 firmware page in red:
3305 Routers with Part Number: 1200880L1 Can Only Run AOS Version 15 or Lower
and there aren't any version 15 images listed so you'll need to get the file from Adtran.