Dear Diana,
This seems really antiquated. I can't think of any discussion groups I belong to that ask you to set the line length of my contributions.
Whatever. Since no one has replied to you, I offer a completely clunky way of doing this. I can't see where Mail has this capability/preference setting.
Anyway, go to barebones software's web site and download the free application TextWrangler. It's just a text editing program. BBedit will do the job too but I'm assuming you don't have that.
In TextWrangler, set up a document with a 60 character width (open a new document, go to Edit-->Text Options... and check "Soft Wrap Text" and under the "to character width" option put "60" in the box).
You can save this document with a silly name so you don't have to create it every time you want to format a mail.
Next, either compose a mail in the document or paste your text into it.
It will be 60 characters wide at this point, but the breaks will be "soft"; you need to go to Text-->Add Line Breaks. Now you can copy your text and paste it into an email.
This sounds like a lot of steps but it is easy. You could probably Applescript it.
Anyway, there is probably a much better answer to your question, but I didn't want to leave you hanging longer. Once your problem leaves the front page it gets less and less attention.
Also, note that in Mail preferences you can find the option to compose and send mails as plain text rather than rich text.
G4 933 mhz Quicksilver Mac OS X (10.4.8) Wacom Intuos 2 tablet; Epson 2400 Photo; HP Deskjet 6840; LaCie 80gb D2 FWDV
