Now I know already people are rejecting what I have to say, but hear me out.
When a designer looks at a blank page either on a computer or a sketchbook, we know the size of the open area we have to work with. We can use the entire page. With Dreamweaver, illustrator, or any design program we have the versatility to go with any size we need.
But with the rich content age here designers have been urged to learn WordPress as a design platform. We are hired by companies and start-ups to design a website that is update-able from anywhere. And yes that is the AWESOME thing about WordPress. From the app on any smart phone, to multiple apps on an iPad; you can update the site from anywhere.
Here is the current issue, as far as art is concerned we (designers) can touch background and the header the most. Where the content lies we have little control over. So when it is time to review thumbnails we can only provide a header and background.
Remember that the website should reflect what you need for your business. If it is content rich then your design can take a back seat. But if you are a creative, actor, artist, perhaps your WordPress should supplement an artistically done website.
Before you sit down with a designer have your marketing and target audience in mind as well as how your site can flow. Decide how you would like to handle the delicate form of art and content. They are both important.