redo log size

[drishti]

any body can tell how to find the appropriate size of redolog files,any formulae,
can we increase the size after db creation.

[Halo]

It is suggested that you size the redo log files in a way that there are no more than 3-4 log switches per hour (at peak usage). Yes, you can increase the size of the redo log files by dropping/recreating them.
alter database drop logfile group x;
alter database add logfile group x ('location1', 'location2') size 50M;