BLKSIZE is the size of the physical block. The larger the block, greater is the number of records for a FB or VB file.
The BLKSIZE parameter specifies the maximum length of the block. RECFM=FBS could be also specified, meaning fixed-blocked standard, meaning all the blocks except the last one were required to be in full BLKSIZE length. RECFM=VBS, or variable-blocked spanned, means a logical record could be spanned across two or more blocks, with flags in the RDW indicating whether a record segment is continued into the next block and/or was continued from the previous one.
The Partitioned Data Set can only allocate on a single volume with the maximum size of 65535 tracks.