There is a document in the Steem blockchain code repository that explains how to setup a full node. The section on "Resources usage" was last modified on July 4th, 2017 and reads:

Shared memory file for full node uses over 65GB

I hope this helps. :)

edit: Actually, the readme has more up to date information:

For a full web node, you need at least 110GB of disk space available. Steemd uses a memory mapped file which currently holds 56GB of data and by default is set to use up to 80GB. The block log of the blockchain itself is a little over 27GB.