Take a look at the Streamlight Microstream and Stylus Pro. Both of them are the same diameter (a little smaller than what a carpenter's pencil would be if it was totally round). The Microstream is 1 AAA, and the Stylus Pro is 2AAA. I used to carry my Microstream in the change (watch) pocket of my jeans. When I carry either one in a jean pocket I can't even tell they are there. There are a lot of other similar lights out there, but I don't have any experience with them. I use lithium primaries for the longer life, but a lot of the members here will tell you to use NiMH, preferably Eneloops. Your choice. Until recently I didn't have a good charger, so I may be doing the switch once I use up most of my current stock.
I think you will find that a true penlight, like the one shown, is designed to be a 'pen' light, and as such, is only good for a couple of feet at most. That's fine for desk work, or reading in your tent at night, but for lighting your path or looking around I think you will like what you will find when you take a step up to lights like the Streamlights I mentioned. Go to BatteryJunction.com and look around. They have a section on 'Best of' and one of the categories is pocket/keychain lights. You will be surprised at the capabilities for the price of some of those lights.