Playlist size can be defined by duration (in minutes); total number of tracks; or the aggregate file size of the tracks contained within the playlist. For example, you can limit your playlist to 60 minutes in duration; request that your playlist not exceed 20 tracks; or restrict the total file size of the playlist to less than 100 megabytes.
The constraint setting for total file size is useful when transferring tracks to a device or burning CDs. For example, if you have an MP3 player with a capacity of 512 MB, you can create a playlist corresponding to 512 MB before transferring the tracks to the device.