It really depends on the smith and how they work.  As a bladesmith I like my anvil a little higher than what is considered standard (top of the anvil at your knuckles).  A lower anvil serves well for striking (sledge hammer work) while a higher anvil serves for more precise work.  You want to be comfortable at the anvil, this will reduce fatigue and wild hammer swings.  There is no one height fits all.

I always have had my anvil at the height of my hammer face when I hold it with my harm straight and standing straight. When I have smaller objects to work on, for instance or have some detail to work on, I hold a steel plate or block in the vice. If few of the latter, I just bend down (it gets uncomfortable when you have too much work to do in that position).

So, I might say that for me when I have to bang metal I get my hammer to travel longer and when I have to be more delicate and tap more than bang, I get the material closer.

Another thing, when at your full extension, you want your hammer face to hit squarely, not with the fore edge, neither with the back edge. 

Hope this helps.

Knuckle height for me in all situations. Its the best all around. A little high on occasions, a little low for others, but just right for most. Hammer control easily solves the problems where you are a bit off. KISS rules'

36yrs ago I was a face about even with hammer face as Yves pointed out.   In the last 5 years I  started to look at most people and it seems their inseam is pretty close to their anvil heights. 

 35year ago it was one of being a professional smith working on all sorts of items mostly by myself.  I had a job that involved some 6ft bars 1.5" sq that needed to have balls forged on the ends..   Those bars were heavy enough that I could not stand at the anvil and keep the bars at a 45degree to forge the balls over the far side of the anvil.

 My arm was not strong enough to stay bent at the elbow lifting those bars up.. So after a couple of hammer blows I'd have to straighten my arm. 

It dawned on me that with my arm straight down I could hold the bars easily, but they would not be at that 45 needed with my anvil at 34" high

The solution was to lower the anvil until I was able to keep my arm straight and just using the arm to pull the bar backward and not up allowed for me to hold the bar back, find that 45degrees and complete the job..  

This became how high the anvil stayed (31") for the next 20  which is 2024.   this is now my normal height and anything higher than 31" I feel as though I"m short cycling my hammer blow. 

I find that I fatigue easier and get less work done.  

My inseam is 34".  If my anvil is 34" I lose 3 extra inches of hammer travel and for me it's terrible.   When I'm out demonstrating or teaching I'll put up with it, but if I know I'm going to be at a single forge station for more than a few days I'll bring a spacer platform to stand on while at the anvil. 

31" for me is ideal, 34" inseam. 

I have the same anvil height for all work..  Knives, axes, hammers. hinges, etc, etc, etc.  my hammer weights also vary from 1.5lbs up to 8lbs in a single hand hammer.. my preference for 90% of forging are 3 main hammer weights of 2.3/4, 4lbs, and 6lbs. 

by the way.  like anything that is physical.. It takes a month to see or adjust to something new.  

So, once you start to spend time at your own anvil and forge on a regular basis try different heights for at least a month.. 

People cut a stump for the anvil and then leave it there..  I was guilty..