What is the current size of txindex folder when txindex=1 set?
27GB as of 2020-06-14 



You yourself can check this via going to .bitcoin/blocks/index folder. Where .bitcoin folder is usually hidden folder on home when using linux. 

It depends if you have pruned than the index folder might appear less in size. As of today my index folder is 105.4 MB since I've prune=1000 and it would remain the same in future as well.

Our question is answered here as well. Please check comment by @anonymous