InnoDB: Error: log file ./ib_logfile0 is of different size

InnoDB is insanely picky about it's config; if something's not right, it'll just give up and go home. To make a change to the log file size without data loss:

1. Revert any config changes you've made to the log file size and start MySQL again.
2. In your running MySQL: SET GLOBAL innodb_fast_shutdown=0;
3. Stop MySQL
4. Make the configuration change to the log file size.
5. Delete both log files.
6. Start MySQL. It will complain about the lack of log files, but it'll create them and all will be well.