Most of the time during that "most of the time," the graphics workload is sufficient for both the GMA HD and the 320M.
As long as the workload stays within the GMA HD (which is most of the time), the 13" will be fine. As cathyy stated, tasks that exceed the GMA HD but are within the 320M may favor the 13". But when the workload exceeds the 320M, the 15"/17" is the better choice. Now, that doesn't take into account power consumption.
What is the use of "better horsepower"? Nothing at all. It does not improve anything. Ever. A better processor, even though not everyone might indeed need it or profit from it, can improve working. (I do not know how else to write this in English, sorry. The word "working" was as close as I could get.) In other words, this topic does not make any sense.
Code:
Low performance High performance
13 320M
15/17 GMA HD 330M