I agree with all the previous posters concerning rod length. I tend to like to fish rods on the long side more for their ease of mending, roll casting, and holding line off the water when nymphing.
With respect to rod weights, I own rod weights from 3-6wt for trout fishing. My 3wt is used exclusively in smaller streams, with small flies, no weight added, and in calm conditions. 90% of the time the 4 and 5wt rods work the best for the conditions I face. I use my 5wts most early in the season, because of higher water conditions (usually), more wind, and fishing larger/heavier nymphs and streamers. As fly sizes and flows decrease in the early summer, I use my 4wt more in both small and medium sized streams. My 6wt is used on big water – the Delaware River and the big rivers out-west. The 3wt is a true specialty rod for me, and rides the bench for most of the season. There’s nothing that a 3wt can do that a 4wt can’t – but the reverse is not true – IMO. Good luck.