1. I want to make a complete copy of my Music library converted to MP3 to play in the car on a long trip. I understand that can be accomplished in Music. Is there a procedure to accomplish that in mass?
2. How can I determine the data size of the library in order to have the correct thumb drive size?
I assume you are using all imported songs or purchased songs, and not using the Apple Music stream service.
In that case, exporting is just drag and drop. Create a folder in the Finder and have that ready. Then select songs in the Music app, and drag and drop them into the folder.
If your music is already mp3 files, then that's all you need to do. If they are something else, like AAC (.m4a) format, then you need to convert them. I'd first make sure that you really need .mp3 files for your car's system. It could be that .m4a files work just fine. Take a few songs on a USB drive and try. Also, try simply taking a few .m4a songs and just changing the name to .mp3. It could be that your system insists on the name of the file being .mp3, but is perfectly capable of playing the AAC format that is typically named .m4a. Cars are funny that way, often years behind.
If you do need to convert them, you can do it outside of the Music app using an app like VLC which has a batch convert function. You can do it in the Music app too, but it is a little messy. You need to set the format to MP3 in Music, Preferences, Files, Import Settings. Then you need to select all of the songs in the Songs view. Then File, Convert, Create MP3 Version. Wait for it to finish. Then sort the Songs list by Kind so you can select just the MP3 songs. Then drag and drop all of those to the Finder folder you have waiting. Then delete the MP3 files to get rid of the duplicates.
There is some danger here too. If you have some music already in MP3 format and some in AAC format, then creating new MP3 copies will mean you have 2 copies as MP3 files. So you want to be careful and not copy the duplicates and definitely not delete both copies when you are done.
Once you have a folder filled with all of your songs, just select the folder in the Finder and use File, Get Info to see the total size. Get a drive a little bigger than that. So if it shows almost exactly 64GB, don't get a 64GB drive, but go one size up just in case.
Thank you simply changing the extension to mp3 did the trick. Doing them one at a time will take forever. Do you have any means of changing the extension on a batch of files at once?
Dennis: Just batch rename them. See https://macmost.com/step-by-step-using-the-mac-batch-rename-tool.html