In PE12, from what I read I believe one would have to stretch a single video clip in Expert Mode without stretching the entire timeline? The only info I can find by searching is involving stretching the time, I want to stretch the viewable portion to fit the screen better, but not the rest of the clip. I already ruined an entire composition by stretching/zooming the entire clip.
I am not clear on your question, so here is what I think you are asking plus some extras
If you are talking about the visible timeline and not the duration of an individual clip, then
a. Click on the Fit to Visible Timeline icon above the Timeline
b. Use the keyboard shortcut to do just that \
You can also zoom in on the visible timeline with the Zoom Out and Zoom In slider above the Timeline.
The slider has 11 clickable positions. The default appears to be 2 clicks to the right of the maximum Zoom Out.
Please let us know if that targeted your question. If not, please more details.