I managed to fix the following via registering my own css in module.config 

  <clientResources>
    <add name="epi-cms.widgets.base"
    path="/css/EpiserverEditMode.css"
    resourceType="Style"/>
  </clientResources>


EpiserverEditMode.css content 

.Sleek .epi-dialog-landscape .dijitDialogPaneContentArea {
    height: 450px !important; 
}
 
.Sleek .epi-dialog-landscape, .Sleek .epi-dialog-portrait, .Sleek .epi-dialog--auto-height {
    width: 700px !important; 
}

Is there a reason for you not adding height: auto instead of 450px?

.Sleek .epi-dialog-landscape .dijitDialogPaneContentArea {
    height: auto !important;
}