Sometimes when you try to start a database, you may get ORA-01200 error like:
ORA-01122: database file 8 failed verification check
ORA-01110: data file 8: '/bitest/oradata/bitest/cists02.dbf'
ORA-01200: actual file size of 64000 is smaller than correct size of 416256 blocks
The best solution is restore if you have a backup.
However if you don't have a backup, you can try to make the datafile have same block sizes registered in the control file to cheat oracle, but you may face data loses.
Steps as below (Caution: use at your own risk!!)