Você pode fazer isso definindo -Ddfs.block.size = alguma coisa com o comando hadoop fs. Por exemplo:
hadoop fs -Ddfs.block.size=1048576 -put ganglia-3.2.0-1.src.rpm /home/hcoyote
Como você pode ver aqui, o tamanho do bloco muda para o que você define na linha de comando (no meu caso, o padrão é 64MB, mas estou mudando para 1MB aqui).
:; hadoop fsck -blocks -files -locations /home/hcoyote/ganglia-3.2.0-1.src.rpm
FSCK started by hcoyote from /10.1.1.111 for path /home/hcoyote/ganglia-3.2.0-1.src.rpm at Mon Aug 15 14:34:14 CDT 2011
/home/hcoyote/ganglia-3.2.0-1.src.rpm 1376561 bytes, 2 block(s): OK
0. blk_5365260307246279706_901858 len=1048576 repl=3 [10.1.1.115:50010, 10.1.1.105:50010, 10.1.1.119:50010]
1. blk_-6347324528974215118_901858 len=327985 repl=3 [10.1.1.106:50010, 10.1.1.105:50010, 10.1.1.104:50010]
Status: HEALTHY
Total size: 1376561 B
Total dirs: 0
Total files: 1
Total blocks (validated): 2 (avg. block size 688280 B)
Minimally replicated blocks: 2 (100.0 %)
Over-replicated blocks: 0 (0.0 %)
Under-replicated blocks: 0 (0.0 %)
Mis-replicated blocks: 0 (0.0 %)
Default replication factor: 3
Average block replication: 3.0
Corrupt blocks: 0
Missing replicas: 0 (0.0 %)
Number of data-nodes: 12
Number of racks: 1
FSCK ended at Mon Aug 15 14:34:14 CDT 2011 in 0 milliseconds
The filesystem under path '/home/hcoyote/ganglia-3.2.0-1.src.rpm' is HEALTHY