As I read somewhere it is either 1/3rd of the log buffer or 1M.
I increased it to this size(1.7M) as the log writes was slow, so I thought as the log buffer is small the 1/3 portion is getting filled up quickly and thereby sending too many quick requests to the LGWR which in turn slows up the redo writes.

I saw some improvement in the redo writes after I increased(doubled up) the size of the Log buffer, but still during log switches or certain high activity it goes up to 1000ms.
So I think the problem might be with IO.

My logfiles and archives are on different disks.