I use a 64GB USB stick - actually two so I just swap sticks over rather than having to go and grab the stick from the car in order to update/add stuff to it:
- this has around 150 CDs of music on it ripped to MP3 at 320K and I've only used up 16GB of space so far.
- it will take the car a few minutes to read the stick the first time you put it in, but if you leave it in it will work instantly next time you start the car.
- the stick needs to be formatted as FAT32. For larger capacity sticks you may need to download software in order to format them correctly (e.g. guiformat). Also be aware there are a lot of fake USB sticks on the market at silly prices that will crash out long before you reach the quoted capacity.
- if you're using MP3s make sure they are tagged correctly. I use a piece of freeware on windows called ID3-TagIT. It's not entirely intuitive, but I find I can fix the Album/Track Title & Number/Artist tags so that I will be able to find the tracks I want when the stick is in the car. I put each album into its own folder to make file management easier.
- Try to keep album names and track titles short because the car display is limited in length.