It is hard to suggest a reasonable image size.
This depends strongly on what resolutions your users use. Small resolutions and big images means a lot of scrolling for your knowledge base users.
About screenshots - It is simple to take a screenshot and use as image in an article, but keep in mind that the Knowledge base frame is smaller than the whole browser. Just pasting in a standard screenshot will result in scrolling for your users.
I would recommend that, if you use large screenshots as images, you scale them down before saving and uploading them. One can often still read text even if the image has been scaled down a bit.
I've sometimes scaled images to 60-80% of original size myself, but it depends on how much detail you need to see. The image scaling/sizing can be done in a simple image editing program, which also has many other useful features like cropping, and putting informative text in the image.
Note: Image compression sometimes increases the file size of the images even if the actual size is smaller.