Doesn't have to be the runway edge per se, just the ground either side of you (so runway width shouldn't matter). The main point is not to look in the near distance because your eyes converge (you go cross-eyed to an extent) and you lose some peripheral vision. I'm convinced this is the reason why 'looking long' helps, though it will also allow you to judge that you're slightly nose-high in the flare so the main wheels touch down first.
If the ground is not flat for a reasonable distance either side, e.g. there's a hedge or a bank near the runway edge, that may cause problems I admit, but at any reasonable licenced field you'll have enough flat ground either side of you.
Tall/short people; maybe a very slight difference. I've got a feeling the difference will be negligible - to the tune of an inch or two. Remember when flying at altitude, the higher you are the further away the horizon is, so the same 'picture' gives you (for example) straight and level flight at cruise power/speed. I should imagine the same works in this case, but on a smaller scale.
Different aircraft types probably would make more difference. A mono wheel is going to 'feel' a lot lower when on the ground than a 747. Many jet pilots feel that they have sunk through the runway before the wheels hit when they get back in a light single.
As I said, when on the ground, make a mental note that this is what being on the ground looks like. When the ground out of the corners of your eyes is a bit below that, you're at the right height to start the flare. If any 'calibration' is needed, that should sort it out for you, regardless of aircraft type.
To be honest, once you've worked out how to land your usual training aircraft you get a feel for it anyway, and wonder why you ever had a problem. When you progress onto different types, you just know when it looks right. I have no problem landing a Navajo or a C152, even when swapping regularly between the two. My 'method' is the way I try to teach PPL students new to the circuit after a few hours. Once they've cracked it, they've cracked it and would probably find it difficult to explain to anyone else how they do it. All I know is that I've inherited students that just can't get the landing right, and this has helped them no end. And obviously I don't just tell them what to do and let them get on with it, I help as required until they realise what I'm talking about so they can make their own judgments about how it should look.
Incidentally, I only just thought of the words: 'point the aeroplane down the runway with your feet and keep it on the centre-line with aileron' as a simple way of explaining crosswind technique. Think that sums it up pretty well. Any comments?