What is the size of the blockchain stored on a user harddrive after pruning has occurred?
You can set a limit for blockchain data by starting your node with
-prune=<n> gives the target size in MiB to use for block files
This limit will only include the blocks and reorganization data. You also need some space for chain state, block index database, and wallet.
I run my node with
-prune=20480 (i.e. 20 GiB) and fully caught up the whole directory has 20.77 GiB or 22.30 GB.
I suggest that you leave a few GiB room for overhead.
Pretty much whatever you configure (with some practical minimum obviously).
See for example: How can I run bitcoind in pruning mode?
Note that with 0.11 you can NOT use the wallet functionality when pruning. With the upcoming 0.12 you can (release candidate out now). Otherwise the functionality remains the same.