I found a way to partially resolve the issue by going to Stream Video Player Options and selecting "fixed" for the "Default Responsive Aspect Ratio" then specifying the dimensions in each video I embed. The downside is that the "responsive" tags seem to get overridden by the default settings.
I have videos that vary in aspect ratio between 4:3 and 16:9. It would be nice if we could specify this for responsive settings at the embed level using the shortcodes but it doesn't seem to work. It always defaults back to "fixed".
I am testing mobile with iPhone 4S.