Where to view app memory usage in iPod Touch?
As reported in this webpage: https://support.apple.com/en-us/HT201656.

You can go to Settings to see how much storage you've used by following: Settings &gt; General &gt; Storage &amp; iCloud Usage &gt; Manage Storage.

In there you'll see a list of your apps, and how much storage each one uses. Tap an app for more detailed information about its usage.