If using this type of stalking bug
http://www.flyforums.co.uk/fly-tying...king-bugs.html
then I'd suggest a #6 at the absolute minimum.
You'll probably struggle to cast well with a #6 though & a heavy fly which sinks at around 8 inches per second, so I'd suggest a 9'6" or 10' #7 would make life easier.
As others have said, using a floater is usual, although I can see some merit in using a clear intermediate line with a 1 to 2 inch per second sink rate.
Just find a likely suspect, cast well ahead of it & then twitch the lure back once it has sunk beneath the estimated depth of your fish. What you're aiming for ideally is that the fly is level with the trout & a foot or so in front of it when they meet, so that you can induce an immediate take, or a short follow & take.
In my experience fish that follow for a distance & then decline the fly are unlikely to take when cast at repeatedly. Fish which react to positively to the plop of the fly hitting the water are almost always capable of being caught.
Good luck!