as far as I see limit for 1 field/value is up to 128 characters, however as Michael noticed (in comments below) there could be many fields, very possibly with total size up to 16Kb.
you easy can do test yourself:
Dim session As New NotesSession
Dim db As NotesDatabase
Dim col As NotesDocumentCollection
Dim Query As String
Dim i As integer
Set db = session.Currentdatabase
For i = 1 To 32000
Print i
Query = Query & "1"
Set col = db.Ftsearch({"} & Query & {"}, 0)
Next