No, it's not possible (yet). Thanks for the suggestion
But adding something like:
skinparam minClassWidth 600
should be easy to implement. A minimum width (rather than a constant width) seems safer to me.
Would this be ok for you ?
We have added two different skinparam, that you can test with the following beta version:
Few tests have been done, so feedback is welcome!
skinparam minClassWidth 70
skinparam sameClassWidth true