Leader length: always a trade-off between castability (short) and getting a good drift (long). 8-10' is very typical for me for nymphs and dries, it varies a little cause I constantly vary my tippet length, but that does the lion's share of my work. Anything else falls under the exception category. And I'll try to describe those exceptions. Streamers, I'll go shorter, in the 7-7 1/2' range. Tight brookie streams, depends how tight, but I can go anywhere from 5' to 9', even with dries. For dry flies, if its midges or tricos or something tiny, I occasionally go as long as 12', but usually only after a 9 or 10' leader was tried and I determined it to be insufficient.
Tippet size (diameter): I like stout tippets for streamers like Fishidiot does. But for everything else, divide fly size by 4 and add 1, except I try to avoid 7x unless I absolutely need it.
Size 10: 3x or 4x
12: 4x
14: 4x or 5x
16: 5x
18: 5x or 6x
20-24: 6x.
Tippet length: This is something that has to be constantly adjusted for the conditions. The shorter it is, the more accurate you'll be, the tighter your loop will be (so you can get it up under that branch or overhang), your fly will turn over better (helping with roll casts, bow and arrow casts, etc), you'll have less knot and tangle problems, etc.. The longer it is, the more curves you'll get in it and thus get a longer and better drag free drift. Also, with nymphing or streamers, the farther up you go with fine diameter material, the better you're leader will cut water, meaning less drag and ability to get it deeper.
So, as a very broad generality: Tight conditions = short leaders, short tippets. Wide open conditions = longer leaders, longer tippets.