The Trip Planner at www.parksflyshop.com has a lot of this information.

Flies will generally be larger here. BWO and PMD are the notable exceptions. The time of year you come will play heavily into what you need. In late July on the Yellowstone I'll very rarely fish a fly smaller than a 14, with #6-12 more common, while on the same river in October I'll have my clients use a 16 Trude as the lead fly with an 18-20 BWO or midge behind it, then have them try a 5" long streamer on the 7 weight when fish aren't rising.

Leaders range from 3' of 12lb Maxima for streamer fishing to 12' 6X on some spooky fish water in the fall. For most of the year we use 7.5' 2-4X leaders on the steeper streams and rivers or when fishing hoppers and 9' 3-5X leaders on the meadow streams. Some shops suggest you need 15-18' leaders to catch fish here, but I think they mostly do this to sell their custom 15-18' leaders --I've never used longer than 12' and have never felt the lack of something longer. I don't carry lighter than 5X until early September at least. I did not use 7X at all last year, and in many places 7X is a recipe for dead fish, as a 2lb cutt doesn't need to do more than turn sideways in medium-fast water to make a 2 minute fight on 4-5X into a 15 minute fight on 7X.