I've just re-read your Q and your Post heading ... Although not necessarily mutually exclusive, there does appear a confusing .. at least in my mind between:
1/- "Adjusting size of video preview window?"
2/- " . . how to change the resolution of the preview "
Size isn't necessarily dictated/directed by Previewing quality. You can get 2 Sizes for each quality . .And THEN you can also "pull" the Preview window as big as the Preview function alllows. THEN you can aslo Simulate ouotput too! So . .what is it you want? . .
Thanks for your answer, Grazie. What I'm trying to do is to set the value for "Display" in the preview window (the "Display" value is directly underneath the "Frame" value.).
I thought there was a way to set this value, but I followed yoursuggestions above and got it to fill in the area - it just required a little moving of the panes to do so.