It can be disappointing spending time or money designing an eBook cover only to find that the dimensions are incorrect and it looks stretched or blurred after uploading it to an eBook publishing site. It can be difficult finding out what the requirements are for you eBook cover from the various distributors so here are the requirements below and my own recommendation.
While the requirements differ for different eBook publishing sites based on my experience as a general rule eBook covers should be at least 1500 to 1,800 pixels wide and have heights that are around 1.5 greater than the width. Therefore if you have an eBook that is 1,700 pixels wide this would make the height 1,700 x 1.5 = 2550 pixels. This makes it easier to downsize to smaller sizes but similar dimensions.
Consider using a high dpi resolution of between 260 dpi to 300 dpi so you can use the eBook for a print paperback version later on. Print versions require higher resolution. Try not to upsize eBooks or they will lose resolution and can end up looking pixelated or blurry. It is better to create a large book cover and then downsize later if necessary. When publishing the eBook you may need to reduce the resolution and size thereby creating a smaller file size.
To find out the dimensions and file size of your eBook simply right click on the eBook file and select properties. This will provide the Dimensions, Width, Height, Resolution and File size.