Hands down, water clarity is the most important consideration for me regarding fly size. The dirtier the water, the larger and darker the fly I use. In water that has 3 feet or less of visibility, I have found that either black or dark claret are best.
Water temperature is also important for fly size; but I found it works a bit differently than many think. A sudden change in water temp I have found is when it is time to either go much smaller or much larger (at least 2 hook sizes, 3 or even 4 sizes is better). If the water temp goes up by 3 or 4 degrees in a 48 hour period, I will drop fly size regardless of the time of year. Likewise, a decrease in water temp by 3 or 4 degrees, I will increase the size of my fly regardless the time of year.
A fly and tactic that I have found to be most effective in clear, cold water during the mid-December to end of January period is fishing a smallish spey fly tied on Alec jackson #5 hooks. And the colors I have found most effective are red, red & blue (a favorite of mine), orange (Glasso's Orange Heron is a favorite), or black & purple (my Night Dancer Shrimp is a favorite).
I know this goes against what most people do when the water temp is down there with the clear, cold water of December and January; however, I have caught more fish on these small spey flies in those conditions than on large flies.
Likewise in summer/fall after the rivers drop to low summer flows and before the fall rains raise the rivers, I use wet flies not larger than #6, with a decided preference for low-water featherwing wets tied not larger than 1/2 shank including tail on #6,8, and 10 low-water hooks. Waking flies I fish on #6,8, and 10 hooks. And exception to these low-water wets in summer is Ally's Shrimp tied on #8,10, or 12 hooks.