What database block size should I use?

It depends on the type of data that will be stored in the database

For example, if you want to create a database depends entirely on the storage of binary data (pictures or something like that)

You need to choose the large size for block

According to http://knowledgebase.progress.com/articles/Article/18293 you can use8K size.
And below is link for Block size tips from Oracle: http://www.dba-oracle.com/s_oracle_block_size.htm

As Fadi Said...
most generic is8k Block size It all depends on the purpose of the database and what kind of data you are going to store...
You can also use multiple block sizes in a database...
let say that you have multiple schemas and application in a single database one application you have all the small tables and data is accessed randomly then you can use smaller block size (it has to be defined in init.ora as well as while creating tablespace) for another application where you have big tables and rows to be accessed then its good to use bigger block size tablespace...
Hope this helps