How can a paragraph be reformatted in Notepad++?

This can now be done (at least in v5.9.8) via the combination of some commands in the Edit menu. Edit > Line Operations > Split Lines will insert a hard wrap at the width of the window. The hotkey for this is Ctrl-I. Edit > Line Operations > Join Line will join lines and has a hotkey of Ctrl-J.

So for a paragraph on one line, if you want to wrap at 60 or 80, resize your window to 60 or 80 characters wide, select the line you would like to hard-wrap, and hit Ctrl-I.

Alternatively, if your paragraph was already formatted to a different width, select all the lines of the paragraph and hit Ctrl-J. Then follow the aforementioned steps and you'll have a newly formatted paragraph!

It's sort of weird.

If you just want to wrap a paragraph at 72 characters you can just go to TextFX>TextFX Edit>ReWrap Text to (Clipboard or 72) width. Which will re-wrap your selected text to 72 characters.

If you want a 61 or 87 character wrap however, you need to copy the text "61" into your clipboard and then do the same thing (TextFX>TextFX Edit>ReWrap Text to (Clipboardor 72) width).

I'm sure Don HO would be interested in a patch for some TextFX Reformat Paragraph stuff, if you were so inclined.

To put it all together, go to Settings > Preferences > Editing > Show Vertical Edge, and check it. If you don't actually want to see the "Vertical Edge," i.e. the wrapping line, select "Background Mode". Next, set your column width by clicking the number after Number of Columns.

Select your text, then hit Ctrl-J (i.e. Join Lines), Ctrl-I (Split Lines).

Edit: If Show Vertical Edge isn't checked, it wraps to the window instead.