Open files in the Tag Editor
In the Tag Editor, open one or more MP3 files that should have resized artwork.
Show the Artwork Size column
To find and select the audio files having too big Artwork Size, show the Artwork Size column. This could be done both via main or context menu Columns / Artwork / Artwork Size.
How to show the Artwork Size column
Copy the Artwork to Clipboard.
Select the Artwork you need to resize. Copy the Artwork to Clipboard via ⌘+C or Edit / Copy menu.
How to copy the Artwork
Open the Artwork in Preview.app.
Preview.app is the built-in image viewer in the macOS. In addition to viewing and printing images, it can also edit them.
Open the Preview.app. In the Preview.app, open the copied Artwork via ⌘+N or menu File / New from Clipboard.
How to open the Artwork in Preview.app
Resize the Artwork in Preview.app.
In the Preview.app, choose Tools / Adjust Size. In the Adjust Size window, enter new Width or Height in pixels, and press OK.
How to resize the Artwork in Preview.app
Copy the Artwork to the Clipboard via ⌘+C or top menu Edit / Copy.
Paste the new Artwork to the Tag Editor.
In the Tag Editor, select the range of Artworks and paste the new image via ⌘+V or Edit / Paste menu.
How to paste the Artwork to the Tag Editor
When you finished, check the result of replacing the Artworks. Save the changes via ⌘+S or File / Save menu.