Hello! The images are responsive and will fill the width of their container. The image size you use should most closely match the most common size of the element. If it is too small, the image will be stretched; too big and it will be scaled down and most likely look fine. You generally want to select a size equal to or just larger than the expected size. This will produce the highest quality results.
If you want the image to not expand to fill the container, you will need to add the image into a container that will restrict its width. For example, you could place the image in a row with three columns. The image would then expand to fill at most one columns width. You could also place the image inside a div with a fixed width. This will restrain the image to the width of the div.