Biggest intruders I tie are 4 inches long, and that's HUGE for me, and I rarely get the chance to use them. 2-3 inch ones have definitely been the best for me, even in freezing cold December temperatures. If you're fishing larger rivers like the stamp or maybe the vedder, than maybe a larger one would work, I just don't find the need. My logic is if a steelhead is so aggressive for a spoon - they'll be relatively aggressive for a fly the same size.
I'll also say keep your intruders simple. Less wraps are better. Mine pretty much consist of a rabbit tail, with some wraps of schlappen and 1-2 wraps of marabou (schlappen underneath helps marabou stay flared and dance). Simple chenille body, and you're good to go.
Personally, I disagree with HOOK on the no weight aspect. I tie mine super heavy, and elect to go with a heavy fly rather than a sink tip. I find it more enjoyable to cast a floating line with just a heavy fly, and I find the fly rides better in the water, and that I have more control over it. Everyone likes a different method though, and I would say there really isn't a right or wrong as long as you're getting it down and catching fish.