This seems to be possible using Styles:
<style>
ActivityDiagram {
MaximumWidth 200
.nowrap {
MaximumWidth 0
}
}
</style>
However I have used a slight fudge because I realised at the end that you wanted the notes to word-wrap, and I couldn't find the syntax for Style to do this (it might well be able to), so I cheated and put the "skinparam wrapWidth 200" back in purely to cover the notes...