hi lehibou,
there is no "auto fit" function in scribus.
there are some scripts in the wiki that go in this direction:
http://wiki.scribus.net/canvas/Adjust_a_text_frame_to_fit_its_contentyou can try them out: maybe the fit your needs or you can adapt one of them for you.
btw, you're request is an indicator of the reason why scribus does not have an auto fit function: it's not clear how each person wants the auto fit function to work...
there are big question opens and what we most need is, that somebody goes through the whole feature and specifies how it should work in each possible case (horizontal, vertical fit; behavior with linked texts, ...)
your -- and other's people -- work is very welcome!
but we don't want to just add a half baked solution...
finally, there are ways to make it easier to place your text in scribus, but without knowing what your needs are, it's hard to give you further hints.
ciao
a.l.e