Paul,
I figured out how to resize the iframe preview for different devices for SPPB.
/components/com_sppagebuilder/assets/css/editor.css
Change
#sp-pagebuilder-view.builder-iframe-mobile{width:320px!important}
To
#sp-pagebuilder-view.builder-iframe-mobile{width:400px!important}
Clear the cache, refresh, and now it renders at a proper width that is actually useable and realistic to the layout on modern phones.
Also, I found an even better free tool Responsive Viewer. It allows you to set up as many concurrent device displays for popular models, or even allows to add your own custom size device. Works for phones, tablets and desktop/laptops.
If the dev team could take integrate some of Reponse Viewer functionality into SPPB, that would really enhance and make SPPB even more awesome.
The whole purpose of using SPPB is for saving time and making great web designs. We need to be able to achieve pixel and layout perfection across all these devices. Please ask them to consider this as a top priority.