I am unable to save a 7.5GB slideshow to a 16GB flash drive. I get the message that the file “can’t be copied because it is too large for the volume’s format.” The flash drive is showing it has 15.5GB free. I want to get the best possible quality since this is being entered in my photo club. Do I need a larger drive? If so, why?
The USB drive you have is probably formatted as FAT so it is also readable by older windows computers. The filesystem FAT only supports files in sizes up to 4GB.
To format the drive so it is readable by both Macs and modern Windows computers, you need to format it in exFAT (this will erase all the data that is currently on the drive): https://smilingmac.wordpress.com/2011/04/03/how-to-format-a-usb-stick-exfat/
Most TVs with USB ports don’t support exFat though.