Glad to help! Let us know if you have any more questions.
If you're using PostgreSQL the size is essentially unlimited. With MySQL it is 16 MB.
There are few exceptions to this rule where we may be using a 64 KB column if you're on MySQL.
I wouldn't recommend storing that much data however. If you're using Elasticsearch, the custom data on the User will be indexed, and Elasticsearch will eventually hit a limit as well.
This guide might be helpful to you: https://fusionauth.io/docs/v1/tech/guides/user-search-with-elasticsearch
If you use the advanced registration forms, you can edit user data now. This was recently released as a premium feature (requiring a paid license; more info here).
More info on setting up the user data editing screen: https://fusionauth.io/docs/v1/tech/guides/advanced-registration-forms/#editing-user-data-in-the-admin-ui