One of the things I'm looking for in a B&W is good, properly distributed, mid-tones. I agree that you need to be able to back off a bit when you're looking at composition and balance, but I don't have a problem with a picture that fills my monitor. By experimenting I've found that sizing a picture to 6 inches in its longest dimension at 240 ppi gives me a file that ends up compressed on LuLa, but goes wide enough by expanding the view that I can view the full image on a black canvas. I use 240 ppi because that's my default resolution once I convert a DNG to PSD and finally to JPEG for upload. I don't know what algorithms LuLa's conversion uses, but everything seems to fall into place once the picture's up. I'm sure there are a number of other ways to do this, but my method cuts the number of steps to a minimum and lets me use a Photoshop action to do the job -- a single click.
Here's what I mean. The result isn't good because I had to convert 72 ppi to 240 ppi, and then let LuLa convert it back to 72 ppi after the expansion to a width of 6 inches. You can't really evaluate the mid-tones because the whole thing is pixelated, but when I bring it up and expand my browser to fill the screen the picture is centered in a black canvas. A perfect setup for examining tonal relationships.