user268: From my last question:
Bitcoin Block Size - What are the rules?
I learned that the block size can vary from a few kB up to a MB. This leads to my next question:
Does the block size effect a miner's hashing speed? (i.e does a very large block decrease hashing speed?)
If so, what are the details of that relationship?

user307: While the block header is always the same size, one component of the block header is the Merkle Root which changes with each included transaction.  The Merkle Root is a hash based on a Merkle Tree of all the transactions of the block.  Creating the Merkle Tree requires 2(n-1)+1 hashes.  The Merkle tree will need to be updated periodically to include new transactions as they occur.
To save computing power in pool mining the pool server computes the Merkle Root and then provides it to all miners in the pool.  In solo mining each miner will need to compute and update the Merkle Root.  Current transaction volume is relatively low so the amount of computing power required to update the Merkle tree isn't significant.  However if Bitcoin ever reached VISA level transaction volume (~4000 transactions per second) computing the Merkle tree would require significant computing power, possibly even dedicated CPU/GPU acceleration just for Merkle tree calculation. 
Information on scalability issues for high transaction volume:
Capabilities of Bitcoins and their place in the future

user108: Only block header gets hashed, and it has fixed size, so the total block size doesn't matter.
From Wiki:Block hashing algorithm

The body of the block contains the transactions. These are hashed only
  indirectly through the Merkle root. Because transactions aren't hashed
  directly, hashing a block with 1 transaction takes exactly the same
  amount of effort as hashing a block with 10,000 transactions.


268: <p>From my last question:
<a href="https://bitcoin.stackexchange.com/questions/1747/bitcoin-block-size-what-are-the-rules">Bitcoin Block Size - What are the rules?</a></p>

<p>I learned that the block size can vary from a few kB up to a MB. This leads to my next question:</p>

<p>Does the block size effect a miner's hashing speed? (i.e does a very large block decrease hashing speed?)
If so, what are the details of that relationship?</p>
 307: <p>While the block header is always the same size, one component of the block header is the Merkle Root which changes with each included transaction.  The Merkle Root is a hash based on a Merkle Tree of all the transactions of the block.  Creating the Merkle Tree requires 2(n-1)+1 hashes.  The Merkle tree will need to be updated periodically to include new transactions as they occur.</p>

<p>To save computing power in pool mining the pool server computes the Merkle Root and then provides it to all miners in the pool.  In solo mining each miner will need to compute and update the Merkle Root.  Current transaction volume is relatively low so the amount of computing power required to update the Merkle tree isn't significant.  However if Bitcoin ever reached VISA level transaction volume (~4000 transactions per second) computing the Merkle tree would require significant computing power, possibly even dedicated CPU/GPU acceleration just for Merkle tree calculation. </p>

<p>Information on scalability issues for high transaction volume:</p>

<p><a href="https://bitcoin.stackexchange.com/questions/1708/capabilities-of-bitcoins-and-their-place-in-the-future">Capabilities of Bitcoins and their place in the future</a></p>
 108: <p>Only block header gets hashed, and it has fixed size, so the total block size doesn't matter.</p>

<p>From <a href="http://webcache.googleusercontent.com/search?q=cache%3aFZ9pUxR3ldEJ%3ahttps://en.bitcoin.it/wiki/Block_hashing_algorithm&amp;hl=en&amp;strip=1">Wiki:Block hashing algorithm</a></p>

<blockquote>
  <p>The body of the block contains the transactions. These are hashed only
  indirectly through the Merkle root. Because transactions aren't hashed
  directly, hashing a block with 1 transaction takes exactly the same
  amount of effort as hashing a block with 10,000 transactions.</p>
</blockquote>
