The length really depends on how you want to use it! If you want one on a stick, I would say 8-10 inches long is good, and a width of about 1-1/2 inches seems to work well. I have one that I use to strop my lathe tools mostly but, I kind of use it for everything! It is on a stick, and it is rough side up, so you can load it up fairly well with any kind of compound! The other side it a fine grit sand paper, so you can kind of spruce up a blade if need be! It works fairly well! If you would want to hang one from your bench or a wall or something, I would recommend about 16 inches! I think the width of 2-1/2 to 3 is probably very good. I think the idea of one rough side and one smooth side it a good idea. That way if you want to use two different compounds on it, you can. I'd use the rough side for you first pass, and the smooth side for the final pass. That should work well!
Tom