RTFM!
infatti sono quasi pronto a scommettere che stai usando dei font che non sono scalabili. dai un occhiata alla guida:
Important notice: the sizing works fine with fully scalable fonts (like the standard PostScript fonts), but might not work well with CM/EC fonts which have two limitations: only a limited number of sizes is available by default (precise adjustments are impossible), and the largest size (25pt or 35pt) is often too small. The CM fonts are now available in PostScript type1 format for free (courtesy of BlueSky/Y&Y), to make them fully scalable, it is mandatory to add \usepackage{type1cm} in the preamble of your document. The EC fonts are also available in type1 format for free (thanks to Vladimir Volovich, they are called cm-super), and adding \usepackage{type1ec} 2 in the preamble will make them fully scalable too. So, if you want lettrine.sty to work properly with CM or EC fonts, you will need PostScript versions of these fonts and one of the packages type1cm.sty or type1ec.sty.
The newly released LM fonts, which should replace both CM and EC fonts sooner or later, are fully scalable, so, if you have them installed, you should use them instead of CM or EC fonts. \usepackage{lmodern} is the command to switch them on (add \usepackage[T1]{fontenc} when composing in one of the western languages other than English in order to get proper hyphenation).
You can also consider using one of the standard PostScript fonts (Palatino, Times, Utopia. . . ), they are fully scalable too!
per favore dai anche una letta alla netiquette. in questo caso, a parte che bastava leggere la guida, se io non l’avessi letta sarebbe stato difficile capire il problema, senza un codice minimo compilabile, le informazioni del log, etc etc.
ciao e buoni capilettera
f