The case isn't much bigger than Pi itself. I've given an extra two millimeters above and below the pi. Just to visualize, take your wallet out and chances are it will be smaller than that. The curve in the top is to accommodate the ethernet and usb ports. If you plan on gluing it to anything I think the outline mismatch looking from the top down will be more of a concern than a curved top or rounded edges.
If more people ask for a straight flat case I might change my mind. There are always the cases on shapeways if you want flat on both sides. They'll just cost more or have the 3D print texture to them.