Pre-allocating capped collection sizes is quite hard. Most estimates turn out right; but once in a while, I regret a production-used cap-size. Right now, the only way to increase size of a capped collection is to allocate space on disk by piping zeros, creating a new capped collection, copy elements from src. to dest. and finally, rename dest. to src.
If there are better ways to do this, please let me know.
If this becomes a feature in the server; that'd be great.