Hm, the answer might partially depend on whether you plan to print or not. If you're sticking with the online medium, I wouldn't go much wider than 700 pixels to ensure that the comic fits the width of most people's screen resolutions. I personally keep mine at 648. I don't pay any attention to the length/height of my image as I don't intend to ever print, but the average is probably 1000 pixels.
If you ultimately plan to print your comics, consider the size you're planning. There's standard (a final product measuring 8x10.5 inches) or manga (a final product measuring 5x7.5 inches). You might check out various printing sites (like kablam.com) to see what their technical specs are for printing so you can easily switch from online to print.
Hope it's helpful?