I am presently using 8GB cards. It's not a function of the capacity, rather of the convenience and necessity. Unless you are planning on shooting long videos, a 4GB card formatted FAT 16 will permit auto-booting and no need for partitions.
So far, I must switch partitions when I want to take the pictures off the card and then manually reload the firmware update to again switch partitions so CHDK will autoboot at turn on. I'm shooting 720P 16x9 HD stereo videos, so I need the larger Class 6 cards.
If all you plan to shoot is stills, a 4GB card will last for an awful lot of shooting...!!