Resolution and size test pics
Each of the photos below was resized with PhotoShop Elements to the dimensions shown and each one had its resolution set to 72dpi. As you can see the best horizontal size is 1000 pixels. Make sure that you maintain the aspect-ratio of your picture when you resize it!!
You do not have to use PhotoShop – any reasonably well-featured graphics program will do, provided it allows you to adjust the size of the graphic and set the resolution.