This is just a guess, since I don't know the details on the BIOS you're using, but I would assume the issue is the size of the memory space request (256 MB). You can find out the maximum size empirically by reducing the size of the memory space request, until the BIOS successfully grants it. The BIOS would have to be modified to increase this maximum.
Hope that helps,
The original design manufacturer (ODM) for the Minnowboard is Circuitco:
Because your questions are specific to the board design and system BIOS, the ODM should have the information being sought.