Is there a length of sheitel that would be considered too long and inappropriate for a married woman to wear - either halachically or from a standpoint of proper moral behavior for a Jewish woman?
From a strict halachic perspective, if "foreign hair" counts as a covering, then it would work even if it's very long. (A more interesting question -- could it be so long as to be considered a "burden" [or likely to become one] rather than "clothing/adornment", which would make it prohibited to wear outside on Shabbos without an eruv? Probably not.)
Many rabbis have complained that sheitels that attract too much attention are missing the point (if still satisfying the letter of the law); I'd figure that could apply to an extremely long one. But there are no formal laws about exactly what kind of sheitel is called "attracting too much attention."