Even 800 x 600 would be considered too small in many cases nowadays because with many modern software applications you cannot squeeze the interface down lower than 1024 x 768. The software designers now expect everyone to be on monitor resolutions above 1024 x 768 so they do not design for less than that size.
Your choice of project size and capture size is one of the most important decisions you need to make for any new project. I recommend you raise this issue with your clients at the first meetings you have when beginning a project and get them to find out what monitor resolution most people in the company or target audience are using. This kind of information should be procurable from their IT department, or if the audience is external accessing their website, then Google Analytics will give you a breakdown of monitor resolution percentages.