Best practices for choosing fs block size on ext3/4?
Short answer:
4096
Longer answer:
Larger block sizes reduce fragmentation, at the cost of some wasted space at the end of the block. Having said that, give the size of most hard drives these days, unless you are storing something like a mail or news spool you should use 4096 as it matches the page size (in memory) with the block size on disk.
If you are storing a mail spool, then you should look at using something like XFS which is more efficient at storing large numbers of small files.