Thumbnail previews do take up quite a bit of space in the Plex metadata folder, but they aren’t the only thing that you need to think about. If you don’t use RAM transcoding, the transcode folder can get quite large if you have a lot of simultaneous transcodes going. Any time Plex needs to change anything in the media, whether it’s a full transcode to reduce bitrate or changing the audio stream or even just changing the container, all of the stream components go into the transcode folder. Transcoding to RAM will eliminate that space requirement, but that’s not the only thing that could be taking up space. Offline sync can use a bunch of space as well. If your brother-in-law decides to sync an entire season of his favorite show in anything other than original quality, all of the files will be transcoded and saved in the Sync folder until he’s finished downloading them. There’s also the PhotoTranscoder folder, which stores copies of media posters, backdrops, cast posters and other images in various sizes and formats to be sent to client devices. This one usually won’t be exceptionally large, but it can still get into the several gigabyte range if you have a lot of media and a lot of different clients.
TL;DR Go with a 500GB drive. Even if you don’t need all the space today, you might need it down the road.