A six weight is a practical general starting point. Fish a small shrimp, nymph or lightly dressed Woolly Bugger on a leader strong enough for the fish and any nearby cover. Let the fly reach the feeding level, then use short movements and pauses.
Watch where a wind-worked margin changes from coloured to clearer water. Cover along that edge before casting well beyond it. If the fly is regularly buried in the bottom, shorten the countdown or lighten the pattern rather than speeding up every retrieve.
Where a fish takes, repeat the same depth and angle before changing anything. Keep contact through the pause; the line can tighten suddenly and a very light tippet gives little margin.