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.

Possible resolution:
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

Specs: http://zabbix.org/wiki/Docs/specs/ZBX-5605