Exchange disk subsystem

I am advised by training and documentation to put the transaction logs on a separate physical disk to increase I/O performance. However, I am having trouble finding more detailed advice. If someone has practical experience on this, could you answer the following questions? Thanks.

1. What are the storage needs of the logs versus the information store? Should I use the same size disks?
2. If the IS is on one mirrored volume and the logs are on another mirrored volume, which one should the OS go on… with the IS or with the logs?