In newer versions of Notepad++, you need to locate the TIDYCFG.INI file in C:\Program Files\Notepad++\plugins\Config\tidy
The INI file has a separate group for each entry on the Tidy menu. A group looks like this:
[TiDy clean Document - nowrap]
The text in brackets is the group name. This is the text that shows up in the HTML Tidy menu. The lines following are keys and values.
Locate [Tidy: Reindent XML] which is at the bottom of the file.
Add a new key of wrap: Set the value to either 0 (to turn of wrapping) or whatever line width you want. (I use 160.)
You can also move this group around in the file to re-order your menu. I put mine at the top to make it quicker to access.
So the top of my TIDYCFG.INI looks like this:
; quick reference: http://tidy.sourceforge.net/docs/quickref.html
; cesky preklad: http://www.pspad.com/cz/tidy.htm
; sample config file for HTML tidy from http://www.w3.org/People/Raggett/tidy/
[Tidy: Reindent XML]
[Sample from Web Page]
If you're editing this file from inside of Notepad++, you need to remember save and close NP++, then re-open it for these changes to show up.