Size depends on what the fish are taking on the water you're fishing. This summer you'll have fun fishing size 6 and 8 foam hoppers that float forever and can get some big strikes when grass hoppers are being blown into the water
In CO though, size 20 and smaller isn't unusual, especially when fishing the tailwaters. What size tapered leader do you use? For that size fly I'd use probably 6x or 7x. Do you know the name of the flies you're using or can you find a picture of them online? Something like a griffith's gnat usually floats pretty well since it's covered in hackle.
Have you found fishing reports online? I've not been to any of the shops down your way but know some of them have online reports. Even if they aren't helpful in person they can't stop you from using the info they put online
Watch the Orvis shop at Parkmeadows Mall in South Denver for their free Fly Fishing 101 class. Not the same as a guide but it's free and helpful.