IIRC, you want somewhere around a 3:1 to a 3.5:1 ratio between your spine and the width of your razor. Equally as important is the thickness (or rather the thinness) of your edge. Some where around .005" is typically a desirable thinness.
You want to made sure that it's thin enough that when you establish your sharpening angle (which as Laurence stated, is relative to width / spine thickness), your bevel doesn't exceed about 1/16th of an inch in width. IOW, if you measure from the point of your edge to where the secondary bevel begins, it shouldn't be wider than .05 to .06 inches or so. Any wider, and you're just increasing the amount of metal that you have to remove when sharpening, which makes things that much more difficult.
Now, you CAN mitigate this (on older razors that have been sharpened a million times, for instance) by raising the spine a little bit and increasing your sharpening angle. This can be accomplished by putting a couple layers of tape on the spine. Obviously this is somewhat of a compromise from a thin sharpening angle, but it makes things a little easier.