How much extra disk space does txindex=1 require?
The total extra size is around 7GB as of November 2016. You can check by looking at the size of the $DATADIR/blocks/index directory.
If you copy that directory, you'll copy the index with it.

Updated values from comments:

11GB as of 2017-05-08 – via
ens
21GB as of
2019-07-16 – via Vasil
Toshkov
27GB as of 2020-06-14. – via
Anonymous