Oracle will fill the redo log before switching. If a manual log switch is performed, the redo log may not be 'full'. If you are archiving, the archived redo log will be the actual size of the redo, not the size of the log. This is probably your best indicator of actual redo generated.
In v$sysstat, the stat 'redo size' is the number of bytes placed in the log buffer. If you divided this number by the number of logs generated since instance startup, you will get a very rough idea. However, unless you performed a complete log switch at shutdown and all the entries had been written to the redo logs and another switch had occurred, your estimation will be off.
If you want to know this to figure out how much Redo is generated every day (or hour) then the best bet will be to look into the Alert file for all the Normal Log switches (switches that were not caused manually)
Count the number for a reasonably long period (a week or a month ) and then devide by the number of days in that peiod to get a rough idea of the redo amount per day.
For accurate results, this assumes that the instance was not shutdown during that period.