Image dimensions and file size are related but not identical. A large photograph may have millions of pixels and still be compressed efficiently, while a smaller image can be large if saved with little compression.
Choose dimensions based on the final display area. Avoid enlarging a small image when sharpness matters. For a website, also consider the device sizes visitors use and provide an image that is large enough for the display but not dramatically larger.
Keep the original image as your master copy. Create resized versions for specific uses so you can change the dimensions later without repeatedly recompressing an already processed image.