Here, 19 (512KB) is listed twice on the end, corresponding to both FLASH_256KB_INFRARED and FLASH_512KB_INFRARED. So I think FLASH_256KB_INFRARED saves would be considered twice the length. Sadly, I don't have "Active Health with Carol Vorderman" handy to test...
I have been staring at the source intently (porting save management functionality to GM9), and I have noticed this line:
https://github.com/TuxSH/TWLSaveTool/blob/2e08b179e8363c7c7c144cd0f6ef1abe4cc12cd3/source/SPI.cpp#L98
Here, 19 (512KB) is listed twice on the end, corresponding to both FLASH_256KB_INFRARED and FLASH_512KB_INFRARED. So I think FLASH_256KB_INFRARED saves would be considered twice the length. Sadly, I don't have "Active Health with Carol Vorderman" handy to test...