If a Cisco router fails to load an IOS because of less flash size/register flag/any other reason, it will stuck in the ROMMON mode.
1. To recover, You should have the IOS image in the flash/external disks.
rommon 3 > dir disk1:
2. Instruct the device to boot the image from the external device.
rommon 4 > boot disk1:ios.bin Loading image, please wait ... Initializing ATA monitor library... Self extracting the image... [OK]