Use ITextSharp To Determine Text Field's Maximum Number Of Characters Allowed?

I have a PDF document with a number of text fields, several of which have a maximum length - namely, a maximum number of allowable characters. Is there a way using iTextSharp to determine this setting? Here's the code I have so far:

Dim reader As New iTextSharp.text.pdf.PdfReader("Foobar.pdf")
Dim inputFields As IDictionary(Of String, iTextSharp.text.pdf.AcroFields.Item) = reader.AcroFields.Fields
For Each key As String In inputFields.Keys
Dim PDFFieldName As String = key
Dim MaxFieldLength As Integer = ???
...
Next

I need to set MaxFieldLength to the number of allowable characters for the current form field being iterated over.