Why it's a fashion trend among Chinese and Taiwanese girl to wear long socks and big size shoe ?

cuz that's what Japanese girls wore a couple years ago. If you want to "predict" the new fashion trend among Chinese and Taiwanese, just take a look at one of those Japanese girl magazines (ie. Nonno) and give a year or so.

it's always odd to ask about fashion for there is never a reason for that... people just wear what they like...

long socks are sexy, see those in porn often.