Take a look at the Michelin Green Guides. They lay things out geographically with a couple maps in t

Hi there! We are planning to travel to Munich and drive to Frankfurt, and would like to drive for five or six days exploring the countryside. Any recommendations on renting a subcompact automatic vehicle from a reputable car rental company would, as well as for accommodations in small towns along the way would be most appreciated. We live in Canada. Thanks.
I suggest not getting too tiny a car . A trunk is nice to hide belongings and the autobahn is not much fun in toy (ie. smart car) car . Not that you want to speed but there is wind and you want to keep up safely with traffic . You will save a lot of money by spending the night a bit out of town both in food and lodging ,and be a bit appreciated. Have fun
We rented a car in Munich from Avis. It was a Fiat Bravo, diesel/manual shift. We drove the Romantic Road (more-or-less) and stopped in Noerdlingen, Dinkelsbuehl, Rothenburg, Nuernberg, and Wuerzburg. We then turned in the car at Frankfurt Airport before our flight home.
If you plan to drive through Germany, Rick's book makes some good suggestions, traveling nurse jobs but he leaves large gaps in the countryside and many of the cities avoided by casual tourists. Granted, some of these places are not worth traveling half-way around the world to visit by themselves, but they can be quite pleasant for a brief stop if they lie upon your route. Supplement your research with another source, like Lonely Planet or The Rough Guide, as these tend to cover larger areas.
You didn't mention how many people need to fit into your car. A couple would have plenty of room AND power in a car like the VW Golf or equivalent (remember, you don't necessarily get exactly the car you ordered but most likely an equivalent). Ask for a Diesel to save on money spent on gas. A bit smaller might work as well.
However, if you insist on an automatic version your choices might be much more limited as they aren't traveling nurse jobs as common in Germany, especially in the subcompact or compact classes. And you most likely will pay a premium for this option.
Take a look at the Michelin Green Guides. They lay things out geographically with a couple maps in the front including possible driving routes. It covers many small places as well as major tourist sites. Rick is limited in the places that he discusses. For example one of my favorite cities between Munich and Frankfurt is Regensburg, not covered in Rick's guide.