Ideal text-to-image ratio: 40:60. Source
Gmail clips all the email content above 102 KB for HTML Emails. Source AMP Emails may go up to 200 KB for seamless rendering.
This is a best practice that can improve the loading speed of the email.
Use tools like Grammarly extension.
Tools like Grammarly can be handy.
Send a test email and verify.
Runs tests at the same time to avoid inaccurate results. Get the definitive subject line writing framework here.