Changing logical sector size

I’m looking for a way to change the logical sector size on a flash. I’m using DC 9.21 with an RCM3100 board. The user manual, in section 11.4, talks about using fs_setup to change the sector size, but no amount of tinkering with values seems to get it working.
In addition, I don’t want to sub-divide the flash into different extents, which seems to be the main purpose of fs_setup. Can anyone give me some direction as to how I might accomplish this?
The end result I’m trying to achieve is to have my preferred flash device have a single 256K logical extent (the full size of the flash) of 1K sectors.