https://bugs.kde.org/show_bug.cgi?id=446020#c5

> [...] we could try starting out with a default size of 800 x 1100
> pixels [...] Perhaps this could be user-configurable?

In Diagram Properties page add input fields for changing canvas width
and height:

umbrello/dialogs/pages/diagrampropertiespage.ui
- In QHBoxLayout containing ui_labelZoom and ui_zoom, at right edge,
  - add QLabel ui_labelWidth with text "Width:";
  - add QDoubleSpinBox ui_width;
  - add spacer;
  - add QLabel ui_labelHeight with text "Height:";
  - add QDoubleSpinBox ui_height.

umbrello/dialogs/pages/diagrampropertiespage.cpp
- In constructor set ui_width from scene->width() and set ui_height
  from scene->height().
- In function apply() call m_scene->setSceneRect() with arguments
  w = ui_width->value() and h = ui_height->value().

CCBUG: 446020