I think a 512GB drive would be more than enough. My drive is 232GB and has 32 games on it and it is like half full.
On my drive, I have two seperate partitions: the first is a WBFS partition and takes up most of the disc, the second partition (100MiB) is FAT32 and is used for homebrew apps. I do not know about Wiiware and VC games as I do not use them. If they are normally stored on the SD card, they will be stored on the FAT32 partition, so you might want to make it bigger.