I would like to include pictures and screenshots. What is the ideal size of an image: length and breadth?
Size your images so that readers can see whatever’s important in them. That’ll vary image by image - eyeball them and also print them out if needed, to make sure that the important bits are legible.
Size-wise, aim for 1/3 to 1/2 page per image; you can go even smaller for images that are cropped.
If you’re creating line art, save in vector format so that you can resize as needed with no loss of image quality.