James,
Sure just do it with css:
.esri-view-width-xlarge .esri-popup__main-container,
.esri-view-width-large .esri-popup__main-container,
.esri-view-width-medium .esri-popup__main-container
{
width: 600px !important;
}
Thanks - I had the esri-popup__main-container, but not the esri-view-width-<size>.
This seems to have been updated in API 4.9:
.esri-popup__main-container.esri-widget {
width: 310px !important;
max-height: 300px !important;
}