This has actually been asked ad infinitum here on ITT..
The answer is that any size SDHC card you can get your hand on, now or in the future, will work (unless it's broken).

The cards come pre-formatted with the correct version of FAT (i.e. FAT32 for large cards), so that's not a problem either. The only issue is if the card for some reason gets corrupted so that it'll need reformatting, if so then you will need to download the free Panasonic SD formatter tool (search the forum and you'll find a link), and format the card from a standalone SDHC-compatible reader/writer (with your PC).