- you can use them to add dynamic tags to your messages (e,g. Dear and the customer's first name).
- you can map them to a import file's columns when importing data into MailUp
- you can use them on subscription forms and profile update forms to collect additional information on your subscribers
- you can take advantage of them when integrating MailUp with other applications
The official answer is complex because it depends on the encoding used for that list.
1) Each recipient field has a maximum length of 200 bytes
2) By default encoding set to a list is UTF-8, which uses from 1 to 4 bytes per character
...in most cases 100 characters work in the recipient field