Where do I set the I/O size for DB2 9.7 LUW driver (AIX 6.1)?
The communication buffer size for remote clients is determined by the RQRIOBLK database configuration parameter (more info in the manual).

However, the statement you quote is overly simplistic and does not make much sense for DB2 configuration. Firstly, DB2 will perform direct disk I/O, so the OS read-ahead setting, if such thing even exists, will not have any effect on DB2. Secondly, DB2 will determine prefetch size, which is what you might call the DB2's answer to read-ahead, dynamically during each query compilation, so the number of pages read with each I/O will be different for different tables and even different queries against the same table. Thirdly, if your query has a WHERE clause, not all that is read from disk will be returned to the client, obviously, so the prefetch size will hardly matter at all. 

You'll be better off, instead of trying to follow some very vague and generic advice, determine the actual bottleneck in your system and address that.