Official comment
@Andrew > If you click on the device itself within rekordbox, it will tell you how big the device is and how much space is available:

The bottom-right corner of each playlist will tell you the size of the playlist and its files:

...but here's the thing - even if you had two playlists totalling 500MB, if those playlists have overlapping tracks (tracks that exist on both playlists), the actual total of audio files to export will not be 500MB. If those audio files already exist on the drive, it won't export them so the space required may be less than the actual size of the playlist to be exported.