TL;DR If you want to support most browsers, then don't exceed 50 cookies per domain, and don't exceed 4093 bytes per domain (i.e. total size of all cookies <= 4093 bytes)

Growing Rails Applications in Practice

Check out our e-book. Learn to structure large Ruby on Rails codebases with the tools you already know and love.

• Introduce design conventions for controllers and user-facing models
• Create a system for growth
• Build applications to last

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Behind the link, you'll find a simple HTML page that offers some cookie tests (how large, how many etc) and an overview of this data for various browsers.

Fun fact: You cannot delete cookies with a key that hits the size limit and has a small value.