I actually would go smaller than normal. If you have a bike that fits well already for commuting, get measuring that bike and start calculating backward.

For example, I have an on-one inbred as my commuter (fairly similar functionally and fit-wise to KM), and I wanted to use mountain drops. So, I learned that my dirt drops had 70mm reach and 100mm. Working from my touring bike measurements, I calculated that I needed a size medium frame (I am on the edge between medium and large) and a fairly short stem to maintain a reasonable reach once I added drops. A huge stack of spacers, and I raised my bars sufficiently to make the drops the primary riding position (standard for mountain drop bars).

I would head over to twentynineinches.com and do some reading on drop-bar mountain bike setup, especially because the KM is a favorite for this setup. If using moustache bar, a few parts may be irrelevant (you won't need to worry as much about sufficient stack, I would think), but it should be an informative read.