It looks like a "Width" property of a user graph doesn't have a clear meaning.
In menu Monitoring -> Graphs it's ignored and uses full width of a display, for screens there is its own "width" property.
Single place where we can see the impact is the graph Preview.
We could replace text "Width" to "Width (for preview only)" in the graph configuration page.
There is also a "Width" column in list of graphs, no idea what would be better to do with it.
This also should be clarified on this page http://www.zabbix.com/documentation/2.0/manual/config/visualisation/graphs/custom