Depends on the bug and type of water...
Slow current & flat water-I fish slow bugs...
Usually a size 2 Sneaky Pete, or a 1/0 blockhead dressed in Sneaky Pete fashion. Sent a bunch of my blockheads in for the warmwater swap recently. They seem to like those big bugs dead drift, sometimes with an occassional tiny twitch. Cast it out from the canoe, mend and let it sit...They can see it, the rubber legs are doing there stuff. If they want it they will come and get it...if no take in a minute or so, pick a good looking spot 25' down river and try again. You want it to look like a big easy meal. Check the rubber legs often..If they get fouled on the hook you will not get a strike.
Fast Current or broken water is a different story...I usually go with a big streamlined popper in the 3-5" long range......Pencil popper, Crease fly, Dalhberg diver, Coffey Grinder, or blockhead with a long marabou tail ripped across the top often works well in those areas.... Doesnt seem to work very well on smooth water though.
Cheers