What you want is this:
# rollover file based on maximum size of 10 MB
agent.sinks.flumeHDFS.hdfs.rollCount = 0
agent.sinks.flumeHDFS.hdfs.rollInterval = 0
agent.sinks.flumeHDFS.hdfs.rollSize = 10000000
agent.sinks.flumeHDFS.hdfs.batchSize = 10000
From the flume documentation
hdfs.rollSize: File size to trigger roll, in bytes (0: never roll based on file size)
In your example you use rollInterval of 2000 which will roll over the file after 2000 seconds, resulting in small files.
Also note that batchSize reflects the number of events before the file is flushed to HDFS, not necessarily the number of events before the file is closed and a new one created. You'll want to set that to some value small enough to not time out writing a large file but large enough to avoid overhead of many requests to HDFS.