2,560 x 1,600 size.
Main aims of cover:
1. Appeal to cozy mystery readers—they should instantly be able to recognize the cover as cozy mystery.
2. Highlight that story contains lots of adorable, furry dogs.
The best covers in this sub-genre are clean, simple, not busy, use good contrasting colors and focus on a few key elements. I've highlighted the three main types of cozy mysteries in attachment "CozyCovers".
#1, 2, 3 all focus on a couple items at the center of the cover, usually related to the murder in the book (e.g. for this book, it would be a coffee pot and vat of coffee beans). This is the type of design i'd prefer to go with, however it makes it very hard to get in the dog element—particularly across a series of books. You could add in a collar, or a leash, but generally readers want to see an actual dog on the cover.
#5, 6 with dog walker and dogs together is a good option, too, and would allow for that them (dogs and walker) to be focus of the cover. Would stress that the face of the dog walker needs to be "off cover" or hidden.
#4 & #7-10 all show dogs in a room, involved in a particular scene. This is an option, but is very busy and would require original art, i imagine.
Warms colors and settings required: On the one hand, readers want a mystery plot that involves suspense and a gradual solving of the crime; they want a story that will keep them guessing about the murder’s identity, and which will keep their mind engaged. However, the ‘cosy’ side of the genre implies that readers want lovable and interesting characters, warm and welcoming settings (such as quaint villages, coffee shops and warm kitchens) and developing relationships.