Ken,

Absolutely not for recovery purposes... The size/time is for performance reasons.. If you cut too many logs to quickyl ie.. lets say your cutting a redo log once every 30 seconds then Increase the size. This obviously begs the question of what is the right time... There's no one perfect answer.... Each cutting of a log does take some time and requires a checkpoint so the idea is to reduce the number of checkpoints by sizing the log files accordingly for each systems load.

Time based does have some impact especially for products like DataGuard which require the log be cut and transmitted.

Redo is redo, RAC should have very little to no impact. Except for potentially a bit more logging due to inter instance SCN synchronization. I've not taken much of a look at RAC from that aspect so YMMV.