Q:

What is the composition of the blockchain database?

From axic's explanation of the structure of the ethereum database, we learned that data is stored in Merkle Patricia tries, usually inside a LevelDB store. It consists of structures such as blocks, transactions, accounts and storage.
How large are these various components of the blockchain database?
And in general, what are the findings of data analysis of the blockchain?

A:

account is a local object to easily track changes on the accounts (with a StateObject db). The size depends on the nbr of transactions you execute with this account. An unused account should be less than 1kB.
The maximum theoretical size for a contract is beyond technology limits, and actually around 90kB. 
The block.Body contains Transactions. The minimum size of a transaction is less than 100 bytes. And there are no limits for transaction and block size. The practical limit is on the gas spent to execute the transaction.
(May 2016): it takes less than 2GB to synchronize the entire blockchain

