[TomTom]

AFAIK the radius is not a behavior flag. The bots will crouch on their own but the placement of the first node inside a duct or low tunnel will influence the success rate. A large radius could be detrimental in that the bot satisfies the navigation step too early before crouching. Another thing to try is tweaking the node lower to the floor and using the walk flag so the bot does not jump going in. Also align the preceding node so the bot goes straight in not clipping the sides.

In general Avoid node radii smaller than 10. It could adversely affect navigation success rates, make the bots look like bots and could trap/delay the bots affecting game play or balance.

Getting the bots to go prone might be possible with a roam with the prone flag but of course that only works if the bot had the roam as its chosen action. Now you can put two disconnected nodes on top of each other for the path away from the roam but in this case it would not work.