These two words can have very close meanings. And I think you can contextually use both in each example you've provided in the question.
Here is a complete reference for "bien" (especially I.B, very close to "très") and "très".
In this sentences, you can say (or write):
Sylvie est petite, mais Sylvain très grand.
Both part of the sentence tend to be told in an equivalent way. So I think "très" is natural here.
Sylvie est petite, mais Sylvain bien grand.
is not meaningless. That is to say Sylvain is really tall and more than excepted (in general, not only compared to Sylvie). Nothing bad here, except that the sentence becomes asymmetrical.
For the second one... this is matter of choice:
Vous êtes très belles ce soir!
... objectively, they are really beautiful (no more).
Vous êtes bien belles ce soir!
... beautiful too, but more than expected (or more than something that can be referenced).
... in fact, as there is "ce soir" which could tend to mean this is not always the case... so choosing a word or another without knowing what is the wanted mean is a bit difficult or subjective in my opinion.