Hi,
Thank you for your inquiry.

Yes, the Open Flashloader only supports page sizes with power of 2.
This is because in the J-Link internal these values are shifted to work with them.

Adjusting this for non power of 2 sizes is on the list, however this will not be supported any time soon.
There are good reasons to only use power of 2 for flash sizes and Atmel/Microchip decided to not
follow this convention for some reason.

Workaround:

The page size is used by the dll to call the flash loader to program a page and providing it with the data.
What the Flashloader actually does is 100% up to the flashloader.
Even though it is a little bit "hacky", it is absolutely possible to set the page size to a value
that is not the actual page size, as long as the Flashloader works around it.

Example: "Virtual" page size 512 bytes, actual page size 264 bytes:
1) The DLL then provides the flashloader with 512 bytes to program 1 "virtual" page (offset 0x0).
The flashloader internally programs actual pages:
1st page: 264 bytes.
2nd page: 248 bytes (16 bytes remaining).

2) The DLL calls the routine again to program the 2nd virtual page (offset 0x200):

The flashloader can calculate the remaining bytes of the 2nd actual page by the start address.
2nd page: program the remaining 16 bytes
3rd page: 264 bytes.
4th page: 232 bytes (32 bytes remaining).
...

As already mentioned this is of course only a work around, but it should do the trick in your case.

Best regards,
Fabian