From http://www.gumstix.org/create-a-bootable-microsd-card.html :

Next, set the geometry to 255 heads, 63 sectors and a calculated value for the number of cylinders required for the particular microSD card. To calculate the number of cylinders, take the 2032664576 bytes reported above by fdisk, divide by 255 heads, 63 sectors and 512 bytes per sector: 2032664576 / 255 / 63 / 512 = 247.12 which we round·downto 247 cylinders.

Just swap out the number of bytes (2032664576 in the example above) with your card's size, and calculate.