Hi Jeff,
The hardware ECC within the C6678 was based on the lower capacity NAND flash devices and doesn't support the higher capacity NAND devices with larger ECC requirements. These NAND flash devices can be used but the ECC calculation would have to be made in software instead of using the hardware in the EMIF16. Example code for a software ECC is included in the MCSDK release. If software ECC is included there isn't a limit to the size of the NAND used. We don't currently have any recommendation for higher capacity parts.
Regards, Bill