I believe I've gotten it with css changes within the style.css of the GP widget.
I've added a max-height and overflow options for x and y instead of the regular overflow option.
.jimu-widget-geoprocessing .input-node, .jimu-widget-geoprocessing .output-node{
max-height: 300px;
overflow-y: auto;
overflow-x:visible;
margin-top: 20px;
}
Doing this I ended up with a horizontal scrollbar on pulldown parameters, so I've cheated and changed that to 99% width.
.jimu-widget-geoprocessing .input-node>.editor-container, .jimu-widget-geoprocessing .output-node>.renderer-container{
margin-top: 5px;
width: 99%;
}
Finally made the checkboxes be full width. They're not in the right spot yet and shouldn't be in style.css.
.jimu-checkbox{
width: 100%;
}