My wife and I have done 2 narrowboat holidays with another couple. We chose Viking Afloat (www.viking-afloat.com) on both occasions because we were advised to chose a boat layout plan that suited our needs and then choose the route. The boat we used on both trips had a double berth cabin at each end with it's own bathroom. The galley/eating area was in the middle. We started with a 4 day short break (mon-fri)from Rugby to Warwick (almost)and then back. The second was a 7 day from Whitchurch to Llangollen and back as mentioned in the article in SlowTrav. This by the way is an very informative article. The Llangollen canal was our favourite as well.
Our boat was 57 feet long (Medway Class). Anything longer might be a little cumbersome. Be sure to plan your trip so you can turn around at a winding hole, marina or some such wide spot or junction. Once you have picked a departure base and route be sure to purchase a Pearson's Guide for your particular route. These are about $20 and are invaluable. They show every lock, bridge, winding hole and most importantly every pub. They also give average times to navigate each section (page). They can be purchased online from the narrowboat company.
The website www.ukboathire.com has lots of information and is the rental site for five different narrowboat companies. It lists all of the routes and boats available. Check out the 6 minute video under "first time afloat".
Please feel free to contact me by email if I can be of any more assistance.