The physical log is at initialization time a part of the rootdbs.
So the size should be at least smaller than the rootdbs. After initialization, you should change the location of your physical log from the rootdbs to another dbspace, like the physdbs.
Set the correct parameters in the onconfig too.
Re: Size of rootdbs and physdbs should always same
The ROOTDBS should greater than the physical log by atleast 30MB (to be safe) if your physical log is to be placed in the ROOTDBS. If you try to place the physical log to a separate space such as PHYSICALDBS, the total physical log size should be the size of your PHYSICALDBS. For best practices it should be separated, separted by disk is much better.