I write whatever I feel is appropriate for my text ads. Some are only one short sentence, while others encompass over 1000 characters. When I read text ads on other websites I don't really care how much text there is if I am being informed about something of interest.
That said, I am professionally interested and the casual websurfer may not have my willingness to read an essay about your featured widget for sale. So keep it short and simple, salesman.
Actually I'm just trying to make sure I've got my bases covered in planning page layouts for a client site, but if I plan for standard banner sizes I think most text ad formats I've seen will work equally well in those spaces...so I guess it's a no-brainer really.
For newsletter text ads, the general consensus seems to be 65 characters per line.
If the ad will appear on the actual site, I usually base the decision more on the space I want to give it on the page, rather than following any set guidelines, such as the Google Adwords size. Slightly longer text ad sizes will bring in a higher revenue, if you are selling the advertising space (as opposed to using it for affiliate ad space or AdSense).
RC confirmed where I was headed, i.e. just planning around IAB standards. I should have mentioned that in doing this for client sites there are always surprises around the corner - so I was just looking for the best way to have all the possibilities covered so that the site layout has "ad zones" that can accomodate any standard size that comes up.