A regular "Text" field type should be fine, and no, you are not limited to 50 characters - that is just the default (i.e. "suggested") value. The max character limit will be different depending on the database type (FGDB vs Oracle vs Shapefile/dbf vs PGDB). I think there may be more elaborate ways to store formatting (page breaks, indents, bullets, etc.) using a BLOB field type, but I don't know much about that. If it's just a "note", I think just a regular text field would work fine though.