Preventing mongodb crashes
The journal will be 3 x 1GB files by default, so really you should just make sure that you have at least 3GB space for the journal when you are starting up.  You can use the smallfiles option to reduce that to 3 x 128MB (384MB) instead, however I would not recommend that for any system with significant volume.

Similarly you can turn off pre-allocation using the noprealloc option but again I would not recommend doing so due to the potential performance issues you can hit as a result.

Both of these options must be configured when the process starts, there is no way to (re)configure them after the fact.