Slide Sorter view has stopped working correctly for some .odp files in Impress 4.4.5. It shows only one big slide – thus making Sorter view useless for rearranging or inspecting many slides. Zoom menu is faded out completely. There seems to be no relevant Tools > Option to set default zoom in Sorter view.
Make sure menu View > Toolbars > Slide View is enabled. This toolbar has the field “Slides Per Row” to set arrangement of the slides. The zoom is set automatically according the width of the window.