First of all you need to install the updated plugin.
You should be able to find information on how to do that elsewhere but the steps basically are: download a zip from Github (you can find that on the downloads page https://github.com/henare/jj-nextgen-jquery-slider/downloads) and upload that to your WordPress install via the plugins page.
I’m pretty sure that will work but I haven’t tested the process ??
Once you’ve installed that version of the plugin there’s a plugin option like any other to resize the images – it should Just Work.