My cartoons are drawn by hand, so they are measured in inches, rather than pixels. Years ago, I read that newspaper comic strips are generally drawn at 4" tall by 13" wide (before being reduced to fit onto the funny pages), so I've been drawing my comic strips at that exact size. When I scan them to my computer, I aim for 300 dpi, which results in an image about 1,200 pixels tall by 3,900 pixels wide. I use Photoshop Elements to resize as necessary for posting to websites, while keeping my original size.
Hope that helps, and good luck with your cartooning! I love it, and hope you will, too!