It says it's an area attack, but it never mentions the size.

In the tabletop game, a flamer template is used to show the area of effect of Warp Rift.

I would suggest, then, that Warp Rift should function akin to a flamer. It should allow a dodge test to avoid (but would not allow mere agility tests, as the power doesn't have any non-reaction-using provisions for ignoring the damage), requiring the target move out of the effect's area if its movement allows...

RAW, personally, I'd interpret it as a Line of Effect, where anything touching the line between the manifester and the 20m range limit takes the damage (barring a successful dodge and move).

Either way? Naaaaasty power. Note to self: Do not make any character EVER an overt enemy of the Grey Knights.