> In the rules tab I just put this: +*.jpg[>40]
> 
> to get all images bigger than 40Kb, but Httrack
> (3.45-4) downloaded also images smaller than 40Kb.
> 
> Is there something wrong in my rule?yes.
1) The filter should be "+*.jpg*[> 40]" In
<http://www.httrack.com/html/fcguide.html> it's "*[" Also fcguide has a space
between the angle bracket and the number. (IIRC the space was required for one
direction.) If it is still required then the quotes are also required.

2) Your rule says image bigger than 40k are allowed, but all files on the
starting site are allowed by default. So the only thing you've changed is to
allow 40k images from ALL sites.
If you want to eliminate small images try -*.jpg*[<40]