Thanks for the suggestion.
In last beta https://dl.dropboxusercontent.com/u/13064071/plantuml.jar we have added a new skinparam.
You can use skinparam minSwimlaneWidth same if you want all swimlanes to have the same width.
Note that you can also specify a value (in pixel): skinparam minSwimlaneWidth 500
In that case, the minimum width of swimlanes will be 500.
Tell us if it is what you were expecting,