What dose local_size make for ddr2 hpcii?

As the topic, what dose local_size affect to ddr2 hpcii? For example, I configure local_wdata as 64bits, what is differences between local_size = 2 and local_size = 4?

According to the specification, local_size specifies the brust number of local_wdata and is allowed to set up to 1024.