Sorry to bring up this old thread but I am having the same "issue"
It's not really my issue, I am not doing anything wrong. Its more of a Microsoft bug/limitation
When you create an InfoPath form, use a text control and set it to multiline, then publish it to a form library, even though its multiline and you can put as much text as you want in it once you submit it to SharePoint it only sees 255 characters. The datatype for this on SharePoint seems to be WSTR (255)
However, if you create a SharePoint list and then create a multi-line column (plaintext) it has no limit, even if you then edit this list with InfoPath. The datatype for this on SharePoint is NTEXT (no limit)
Why? It does not make sense.
Is there any way to have a plain text box in InfoPath form (library) and not have a limit when it comes to SharePoint.
If you open the form, all the text is still there but SP only shows 255 characters and consequently if you export the entry only 255 char show up.
I know you suggest to use a Rich Textbox but recently it has become my most hated control in InfoPath.
There has been many many intermittent issues with forms because of Rich Text fields. Sometimes they seem to clear themselves when you tab from one to another. Sometimes they clear themselves when you try to submit. Sometimes they cause weird Javascript errors in IE. I am not doing anything special either, no code, no rules.