[Qgis-user] proportional symbols

[G. Allegri]

I migrate here a discussion started in a ticket
https://trac.osgeo.org/qgis/ticket/960

I change the question asking the following: how to make proportional
symbols, i.e. with the size based on a certain attribute?
I thought I should use the "Area scale fields" in the Properties
window. It does the work, but it doesn't scale the sttribute values,
so if the they are "big" the symbols overlay each other even covering
the entire screen.

I show two images about this:
Property dialog: http://www.geospatial.it/allegri/property.png
Result: http://www.geospatial.it/allegri/graduated.png

I've used the lower allowed "Dimension" (=3)...

Am I using a tool that wasn't supposed to do this work? Or am I using
it in the wrong way?

Giovanni