I can't really understand the shard key concept in a MongoDB sharded cluster, as I've just started learning MongoDB.

Citing the MongoDB documentation:

> 
> A **chunk** is a contiguous range of **shard key values** assigned to a
>  particular shard. When they grow beyond the configured chunk size, a
>  mongos splits the chunk into two chunks.
> 
> 
> 

It seems that chuck size is something related to a particular shard, not to the cluster itself. Am I right? 

Speaking about the cardinality of a shard key:

> 
> Consider the use of a state field as a shard key:
> 
> 
> The state key’s value
>  holds the US state for a given address document. This field has a low
>  cardinality as all documents that have the **same value in the state
>  field must reside on the same shard**, even if a particular state’s
>  chunk exceeds the maximum chunk size.
> 
> 
> Since there are a limited number of possible values for the state field, MongoDB may distribute data unevenly among a small number of fixed chunks.
> 
> 
> 

My question is how the shard key relates to the chunk size.

It seems to me that, having just two shard servers, it wouldn't be possible to distribute the data because *same value in the state field must reside on the same shard*. With three documents with states like *Arizona*, *Indiana* and *Maine*, how data is distributed among just two shards?