In an email I got from Cook's Illustrated announcing the stand mixer tests, the pertinent part reads as follows:
Equipment Corner: Standing Mixers
We've just finished a marathon evaluation of standing mixers that included 18 models (one cost $1,500!) tested on everything from whipping cream to making cookie dough. The toughest test? Our rustic Italian bread, which requires 15 minutes of kneading: Six mixers failed to complete the task, and shaking, sputtering, and smoking (from the overworked motors) abounded. Even more surprising was the poor performance of some of the 7-quart models, equipped with monster-size whisk attachments that left the two egg whites we tried to whip completely undisturbed. Our advice: A 5- or 6-quart mixer is plenty big--but still small enough to beat a few egg whites or whip a small amount of cream. For full details on our tests, look for the November/December issue of Cook's Illustrated.