Have you looked at the man page for dd?

Google is your friend:

dd(1) - Linux man page

For answers to commonly asked questions, please refer to the
InstantCake Instructions and Release Notes,
Official InstantCake Discussion Thread, and the
Official PTVnet Discussion Thread

Hello mrrjm,

Unfortunately, that's probably not going to make much of a difference. The bs flag specifies how many bytes at a time dd should attempt to read/write. While setting it to higher may make a little difference depending on how the dd program is coded (how much overhead there is in starting/stopping), what your total transfer mostly depends on is how fast your disks are and your transport (network and/or internal computer bus).

HTH.

RoboCommunity, home of Robosapien and friends