Change gear on a shimano Schwinn Ranger, not sure what Allen key size to use? I've tried an Allen key 7/64 but that didn't work and the others on my Allen key set but none seem to fit this shimano on a Schwinn Ranger.

One of the derailleur cogs broke in half.

Here is an image of a similar bike with the same Shimano drive train.

http://www.target.com/p/schwinn-mens-ranger-26-mountain-bike-red/-/A-15287654

Thank you!
The allen is almost certainly metric.  Both 2.5mm and 3mm are reasonably close to 7/64 inch.  3mm is a very common size on bikes.  If you are going to do any work on a bike you need metric allen wrenches.