Wal44
Newly Enlightened
I've had a Wuben (check Amazon) on my key ring for almost two years now and it has worked well. Rechargeable 10180 battery lasts about 6 hours on low (10 lumens) and maybe 20 minutes on high (130 lumens). Unscrew cap and recharge with usb. 1.6 inches long by 0.5 inch diameter. Its a twisty so won't accidentally turn on. Might be a little larger than you want though.