Incoming mail list so small I can barely read

Dumping Gmail and strictly using Apple systems now. However, the incoming list of emails font size is too small and I can barely read. I went to "Mail" then "Preferences" then "Fonts and Colors." There is a little pop-up where "Message List Font" is NOT clickable :/ What to do? I don't want to go back to Google.

What's the fix?

Thank you.

Julie