How big can a cookie be?

The answer

RFC 6265 says browsers should support at least 4096 bytes per cookie, counting name, value and attributes, at least 50 cookies per domain and at least 3000 in total. These are minimums that servers should plan around, and real limits vary by browser, so test the browsers you support. MDN describes about 4 KB per cookie as the usual maximum. Keep cookies small, because they travel with every matching request, and store bulky data on the server.