How do I clear all files from my iPod Nano (6th Gen) and start over?

When your iPod is connected to your MacBook, open iTunes, click on your iPod in the sidebar, and in the "Summary" tab, click "Restore iPod."

If you don't see the sidebar in iTunes, go to the View menu in the menu bar, and click Show Sidebar.

Connect it to your PC.

Click 'Recover device'

It's not based on "minutes." Songs (and other media) are encoded differently. A higher-quality encoding (with a higher bit rate "kbps" number) takes up more space compared to a lower-quality recording, for the same song. For example, podcasts and audiobooks are often recorded at a lower bit rate, so it takes a lot more "minutes" of a podcast or audiobook to take up the same space used by a typical song. Also, if you have photos on the iPod, they can take up space on the iPod that is not related to audio content. (A 6th gen iPod nano cannot play videos, so you won't have any video content on the iPod.)

To get a snapshot of the iPod's current content, select the iPod in iTunes (click its device button on horizontal bar) to show its settings screen in iTunes window. At the bottom of the iPod's settings screen, there is a Capacity Bar. It shows, at a glance, the types of media on the iPod and remaining free space.

If you want to "start over," do a Restore on the iPod using iTunes. This erases the iPod, reinstalls its software, and sets it to default settings

Use iTunes on your Mac or PC to restore your iPhone, iPad, or iPod to factory settings - Apple Support

Not knowing about encoding, I removed a number of playlists representing what seemed like enough "minutes"; when that didn't work

Removing playlists does not necessarily remove the songs. Playlists are just lists of songs, not the actual songs. If the songs are on the iPod for another reason, such as being on a different playlist (that was not deleted), or being selected for syncing as part of an album or artist song list, those songs are still on the iPod.

How you remove songs from the iPod depends on the method you used to load them. Please describe how you load songs and other content on the iPod. Do you use the Manually manage setting; you "drag and drop" items from your library to the iPod in iTunes. Or do you set up the iPod's Music settings screen (in iTunes) to tell iTunes how to automatically Sync Music to the iPod.

If the checkbox for Manually manage music is checked on the iPod's Summary settings screen in iTunes, your iPod is currently not using automatic syncing. Here's how to removes songs (and other content) manually.

Select the iPod in iTunes (click its device button on horizontal bar) to show its settings screen in iTunes window. Along the left side (in the sidebar), under On My Device, select Music. To the right, the iPod's song list is shown. Select a song you want to remove and press Delete on the keyboard. You can do this with more than one song selected at the same time. If you have other content to remove, such as podcasts, in the sidebar, select that category under On My Device. And do the same thing.

Keep an eye on the Capacity Bar at the bottom of the iPod's settings screen. Once you have enough free space to fit those audio files, you can try manually adding those files again.

NOTE: Not sure what you are describing about playlists, but IF the iPod is set to Manually manage music, removing playlists from the iPod definitely will not remove the songs. Songs can be on the iPod by themselves without being on any playlist. So, removing the playlist just removes the playlist. The songs stay on the iPod.

If you want a better way to manage what is on the iPod, you should set up automatic syncing using playlists to select what is loaded on the iPod. Then, when you want to change the iPod's songs, you can just update those playlists (in your iTunes library) by adding and removing songs. The iPod does not need to be connected. The next time you connect the iPod, iTunes automatically updates the iPod with the same changes. If you want more info on how to set this up, please post back.