I'd have to say that we don't have enough information, but here's what I'll consider as for adding comments to records.
Is it possible, or likely, that you'll eventually want more than one comment per record? If so, you want a new table with a foreign key to the "master" table. That way multiple comments can be attached to the same record.
As an example, we have a CustomerNotes table. And there are different note types. Different types identify if the note should show up in the Point of Sale system, the CRM, etc. Also the note type can determine urgency.
So we have three tables (in this example).
Customers (CustomerID int, ....)
NoteType (NoteTypeID int, ...)
CustomerNotes (CustomerID FK, NoteTypeID FK, Note Varchar(255)...)
All that said, the length of the note depends on the business requirement. Don't try to decide for yourself. Talk to the business users who will be entering the notes. Figure out what they require. If you can get by with less than Varchar(MAX) do it. If you can't make it MAX.
Also, If it is guaranteed that it will be no more than one note per record, go ahead and add a column and make it nullable.
Hope this helps a bit.