Hello Uday,
I'd use the ObjectSpy to find out what the set limit is for the textbox.
http://support.smartbear.com/viewarticle/11926/
Otherwise you could write a loop function to keep appending a single character to the textbox, read the data from the textbox and check the length. Once it's hit the limit the length would remain the same.
For questions 2 and 3, I assume if you enter in invalid characters, these will be ignored by the textbox. If that's the case, record yourself entering data, then read the textbox and compare with the expected results.
eg. I've used a Windows 7 machine with the Notepad Save As dialog box open for this example. This textbox is alphanumeric but it's just to show you what I expect if this was a numeric only textbox.
Sub ForumTextBox()
'Win 7 Notepad Save As dialog box -> file name textbox
Set NotepadTextBox = Aliases.NOTEPAD.Window("#32770", "Save As", 1).Window("DUIViewWndClassName", "", 1).Window("DirectUIHWND", "", 1).Window("FloatNotifySink", "", 1).Window("ComboBox", "", 1).Window("Edit", "", 1)
'Textbox limit
Log.Message("Textbox limit: " & NotepadTextBox.wLimit)
'Set text as whatever
Call NotepadTextBox.keys("123abc4")
'Read the textbox.
strText = NotepadTextBox.wText
'Check the content - On a numeric only textbox, the letters would be ignored.
If (strText = "1234") Then
Log.Message("Passed")
Else
Log.Error("Failed")
End If
End Sub
Cheers,
Jackson