how to enlarge the text in the "email subject/header plane"

in Thunderbird email, the subject of the email and the email message are in to different planes. No problem enlarging the "message text, words" in the "body" of the email. How to enlarge the "text, font or words" of the "subject" in the subject/header plane? Don't want to open email if I'm not sure of it, and can't be sure of opening emails if I can't SEE the subject of the email. Surely I'm not the only one with this problem.