What is the current blockchain size in gigabytes and is there a service that regularly reports this statistic?
The current blockchain size is around 50 GB as of October, 2018.
On Windows platforms, you can check the size of the blockchain by checking the size of
On Linux/UNIX platforms, you can check the size of the blockchain in the terminal by executing
du -sh ~/.bitmonero/lmdb/.
The Wolfram Warptangent v0.10.0 release bundled in some rather significant database optimizations started by the late developer warptangent (continued and finished by hyc), which shrunk the blockchain down to around 7 GB from 17 GB back in summer 2016.
The Monero blockchain grew fairly quickly in 2017 due to rapid adoption. An optimization known as "Bulletproofs" which significantly reduced the size of all subsequent transactions (~80% reduction) was activated on October 18th, 2018 with the v8 hard fork.
To check the current size of the blockchain (as well as look up historic growth of the blockchain), see https://moneroblocks.info/stats/blockchain-growth
The blockchain database size is currently 21 GB.
Historical evolution is tracked at http://moneroblocks.info/stats/blockchain-growth. This site claims a smaller size, so it might be tracking the "raw" size, rather than the on disk database size (as the database duplicates some data in order to allow faster lookups).
Monthly Blockchain Growth:
Started April 2014. As of December 2017 it shows as 26 GB.
There are two resources that report the blockhain size and growth over time.
The second website loads a lot of information in the browser, so use a desktop browser only and be patient.