I created a new record today (19989553), and added a bunch of info into the prospect tab. I'm trying to add more, but can't type nor cut and paste info into any of the text fields. Much sadness. Are there rules about max character limits, or other constraints that I'm not aware of?
Bad news - there is a character limit.
Good news - you can edit it.
I just edited the limit on the Biography field here
You can see on this field it it has a maximum length of 255.
If you set the limit to 0 it means no limit - at least on the note fields (ie. see the data type here civicrm/admin/custom/group/field?reset=1&action=browse&gid=5 ) - I don't think doing that is a problem for the database. The limit seems to be primarily in the UI for note fields but in the database for 'alphanumeric' fields