USB flash drives would generally be well under the 500mA maximum. Hard Disks sometimes "cheat" the spec so for power alone, I wouldn't plug in hard drives unless they are self powered or you have a powered USB hub. Still, there's not much reason to use a hard drive given how small the files are. Most any small size USB flash drive will work. It's most important to ensure there are no additional partitions on the flash drive. Some manufacturers "like" to put extra software on their sticks and divvy up the stick into multiple partitions.

It looks like the largest size in the compatibility list is 32GB - although I didn't look up every single one. Here I haven't always paid much attention to the size outside of always trying to buy the smallest one available. I may have purchased a 64GB one recently. I'll have to check what size it is. I think the OS will be able to resolve larger sizes.

Per standard, FAT32's largest partition size is 2TB and Windows only supports formatting FAT32 to ... drumroll ... 32GB despite the fact that the spec allows for larger partition sizes.

The hard limit is going to be 2TB although the actual limit may be less.