Yeah, you'd probably think it would be a power of 2 in size. But then again, Mac firmwares have been known to be bigger than their PC counterparts. I'd be curious to know the size of the flash chip on the card. Can you take a picture of it and/or type the markings on it into here? If it's 128KB in size, then we'd know for sure 
When I run the "file" command on the firmware file, it tells me:
ROMFILE.1S2: BIOS (ia32) ROM Ext. (64*512)
What does the PC flasher do when you try to flash it? Also which PC flasher did you try? I've found UpdFlash and SiFlashTool on the Silicon Image website. SiFlashTool seems simple enough to figure out because it's a command line utility. I have to be honest -- I'd have more luck figuring out how to patch the WIndows flasher to force it to work than the OS X flasher, simply due to my lack of experience disassembling PowerPC assembly, and I have access to some nicer disassembly tools that don't work with PowerPC.
When I run the "file" command on the firmware file, it tells me:
ROMFILE.1S2: BIOS (ia32) ROM Ext. (64*512)
What does the PC flasher do when you try to flash it? Also which PC flasher did you try? I've found UpdFlash and SiFlashTool on the Silicon Image website. SiFlashTool seems simple enough to figure out because it's a command line utility. I have to be honest -- I'd have more luck figuring out how to patch the WIndows flasher to force it to work than the OS X flasher, simply due to my lack of experience disassembling PowerPC assembly, and I have access to some nicer disassembly tools that don't work with PowerPC.