You could zoom in the studio and then click "File" and "Export to PNG" and then use that.
Alternatively you can go to KDP and use their Print Cover Calculator and Templates.
Just pick from all the drop down menu's your choices, and enter the number of pages your book is, or will be (this is very important to do, as you can not just guess and say 100 pages, then your finished book is 150 pages, because the 150 page book will be thicker than one that is 100 pages, therefore the cover needs to be larger).
Double check your choices, press "Calculate Dimensions" then "Download Template.