1) A smaller ponytail over straight hair, like I questioned about in forum #69407 already (post #1065614 is another example in addition to the characters I mentioned there).
1)I can't really think up a good name for this one. Even two side up and one side up sounded a bit silly to me, but since they were japanese concepts we went with them. Might I suggest an umbrella tag for all these hair up hair down hairstyles? /huhd + ponytail would cover this, /huhd + side ponytail would get one side up, /huhd + twintails would get two side up.
I would be OK with an umbrella tag if it didn't replace the more specific tags (two_side_up should definitely stay, at the very least). That might be helpful for ambiguous instances, though, where we can see one side of the hair clearly but not the other and there is no canon to reference.
large ponytail will probably be misused to indicate length.
Yeah, I'm going more for width here. I think something like post #623848 would be an example for a shorter-length ponytail for instance. "As wide as the head" might be a good rule as you mentioned.