Right now there are a couple ways to set the "usePowerOf2" user flag on a collection.

1) Create the collection. Then later call

db.runCommand( { "collMod" : coll ,  "usePowerOf2Sizes" : true } );

2) At creation time, run

db.runCommand( {"create" : coll,  "flags" : 1 } );

The latter is not very intuitive, since it relies on the user knowing how to interpret the "flags" bitmask.

We should allow a syntax like this:

db.runCommand( {"create" : coll,  "usePowerOf2Sizes" : true  } );

and more generally, any option you can pass to "collMod" should also be something you can pass to "create".