Currently this function is unavailable in BP, however Set Source Radius is available.
As far as I remember Set Source Length was added later than radius so probably someone just forgot to add access BP(Construction script in particular).
Illustration of my problem, as you can see - light capsule can’t be rescaled 
Hmm that’s odd, can you not drag off of the Light, get the Light Component and set it through there either?
Also try with context-sensitive OFF, I’ve noticed that the Context menu is getting shorter and shorter. A lot of functions are completely missing when you have CS on even though they’re perfectly valid.
Still unable. I have Get Source Radius, Get Source Length and Set Source Radius. Well, everything except this single node 
Hey Guys -
We have a feature request in for this functionality in Blueprints, it is currently backlogged, but I have added your names to it and increased the community interest.
Thanks -
Eric Ketchum
Thank you very much, Eric! =]
Just noticed - this has been added in 4.9, thanks guys! <3