Basic steps are if you have currently 3 log groups, create 3 more log groups with the desired size using
ALTER DATABASE ADD LOGFILE GROUP x
'/usr/oracle/dbs/log4ORCL.dbf' size xxM;
Shutdown database and open it in restrict mode and use the command ALTER SYSTEM SWITCH LOGFILE
and make sure the log file you are trying to drop is not having the status 'CURRENT' and drop it.
See the following metalink doc for detailed steps on how to resize your logs.
I have mentioned assuming there is lot activity on the database as logfiles are small and will be filled so quickly, to avoid the redo creation during the time new log groups are created. Be careful, if you are doing while the database open for users.
Make sure, you switch the logfile to the log group you create with diffrent size before attempting to drop the existing log groups and see all of the log groups you created havng status are inactive/current but not 'unused'.