Currently, to set the WiredTiger cache size in MBs, rather than GBs, we're forced to do so via a commandline option (which isn't listed in the docs, or via mongod --help). This makes the assumption that nearly everyone using MongoDB has or needs large server requirements to work with, and ignores users who run services/sites/application from smaller setups (in particular, VMs).

Also, as mentioned, the command arg to limit the size to MBs isn't listed, and I found it on the mongodb-user google group page:

https://groups.google.com/forum/#!msg/mongodb-user/V2Q4LZAe2m0/pqNSu4psDQAJ

Specifically: --wiredTigerEngineConfigString="cache_size=xxxM"