You can customize the cover of your portfolio and of your standalone collections. Either by selecting a color or an image.
Your portfolio/collection is based on responsive design, meaning it will flexibly adapt its layout to look great on any screen size, no matter if it's a smartphone or a large desktop monitor. This also means that the size and aspect ration of the cover area will change depending on the device.
Example of header image with a standard desktop screen resolution:
Example of header image with a very large desktop screen resolution:
As you can see in the examples above, any image that you upload will be displayed in "cover style", meaning it will cover the entire cover area depending on the screen resolution. The height goes up to 488px max while the width is fully dependent on the screen resolution.
That's the reason why there is no straightforward image size or aspect ration to recommend: On small screens, the image will be resized to fit the smaller screen real estate and on large screens it's the other way around.
What you can do is either upload a picture with a pattern where it doesn't matter which part of the picture is shown and how big it is. Or you can upload a picture that works well for the most common screen resolutions. Example: If you want your portfolio to look great on a screen that's 1512px wide, upload a picture that's at least 3028px wide and 976px high (twice the resolution is advisable so that it will look good on high-res screens, too).