Getting field length in libpq

I have following problem:
I need to get the length of field after executing query. For INT4 fields
it is simple (call PQfsize and get 4) but when I have VARCHAR(30) field
PQfsize returns -1. I cannot call PQgetlength for example first tuple
because this tuple can have 10 chars and PQgetlength returns 10 but I
need 30. Have you any solutions?

BarTeo