You can only preview as much as you've downlded; eg: you downld 2% of a 100 min video, then you'll only be able to preview 1st 2 mins.

Keep in mind that when you preview a video it created a preview file in the incomplete folder which should later be deleted (has the name PREVIEW in front of it.) This preview is a snapshot of the moment you 1st previewed it. It will not change in size. If you later do a preview after the file has downlded more than the size & length will be greater. Preview files are not shared & do not have a corelation to the source & so might be coloured red. An incomplete file sometimes takes on the size of the complete file despite it's not totally downlded (usually seems to happen with videos.) Just follow the downld % you see in the downld window.