we rented a Ford Fiesta for our week in Germany about a month ago. it's even smaller than a focus but it was fine on all the roads including the Autobahn.

We had no problems with being overtaken by cars going faster than us; my only complaint was being "cut up" by cars swerving in front of us from the left-hand lane - they tend to get pretty close.

if you are buying a map, do make sure that it's as up to date as possible. Even the Michelin ones we had that were published in 2009 and 2010 were a bit out of date here and here.

I'm the stereotypical little ole lady from Pasadena, but I have no trouble driving on the Autobahn. As others have said, keep to the right. That's not just a suggestion, either.

Once you get a decent map here at any gas station, you will see that almost any autobahn got a federal highway following same direction. The highways are not necessarily always scenic. You can identify highways which are supposed to be more scenic on maps as they have a green line following the route.
But it makes no sense at all to avoid the autobahn at all costs when you need to get from Frankfurt airport to Würzburg until you get past the Frankfurt conurbation.

A refinement on the above remarks about passing:

Do NOT pass as you would (regrettably) in the US.

When you pass, speed up to pass to enable the entire passing action to take about 5 seconds. The unforgiveable sin on the autobahns is to pass the other car going 3 mph faster. Doing that will earn you flashing lights, horns, and fingers.

Speed on the autobahns was not nearly as stressful as meeting other cars coming the opposite direction on very narrow streets in small towns ... even at 30 kph or 50 kph.

Thankfully, both me and the other drivers seemed to know EXACTLY where the sides of their cars were ... it seemed like sometimes there was only 6" to spare between my car and the builings, and between both cars, and between the other car and the buildings.

Just got back from driving a rental car for two weeks in Germany. IMHO, the autobahns were a snap, because nobody sat in the left lane blocking traffic. We found it hard to decipher the Michelin maps - hard to see the route numbers. Also, many smaller German highways were not marked with the road numbers, hence making it hard to navigate. Personally, I wouldn't drive in Europe without a GPS system.