I do not remember what I have put, may be 0.1 uf, also I may have done a post about it, and a scope was a good friend.
I had a VR sensor (180Ohm) I did a computation to get the cut frequency 10 times the highest trigger freq.
The VR sensor is a nice thing because the highest the frequency the highest the level, hence the signal after the filter may remain high enough.
I might start from 1uF and then make smaller if necessary