Since this is a very asked feature, we have patched PlantUML so that the !pragma directive allows to specify an aspect ratio.
You can download : https://dl.dropboxusercontent.com/u/13064071/plantuml.jar
It was much simplier than creating a new command.
So for example, you can use:
@startuml
!pragma svek_trace on
!pragma aspect 2,5
[component1]
[component2]
@enduml
The "!pragma svek_trace on" line will create svek.dot/sveg.svg file on your machine so that you can check the result.
Tell us if it improves your diagrams, so that we can eventually add a real command (instead of using pragma).