Hi BBQ Brian,
The activity comments are a short summary of the activity itself. The idea is that you summarize what is discussed in the email so that it is easier to locate a specific activity at a later date. While you can copy the entire email and paste it into the comments section, there is no way to increase the number of characters that are auto filled into the comments from the gadget.
You might be interested to know that when you create an activity from the Gmail gadget, it logs the entirety of your email thread. You can see this in the activity you created on the right hand side.
With this workflow, you can easily locate the activity in Method based on the comments. If you need to see additional information all you need to do is open the activity. I hope this helps.