Matteo,

> Is there any kind of limit in the size of the fields of a CONSTRAINT
> UNIQUE?

not exactly, but there is one on the size of an index. A UNIQUE
contraint is implemented by means of an underlying index, which has a
maximum "key size" of approx. 240 bytes.

To work around your problem, short of waiting for a future release of
Firebird, I don't see much you can do. Perhaps you could split your
index in two or three (non unique) slices, and then enforce uniqueness
by means of a trigger or two.
This has some disadvantages, too.
Ciao
--
____
_/\/ando