Can the clip preview in the upper left be made bigger?
Nope. But you could use Icon View (Ctrl+Page Down) and set the Thumbnail Size to Large.
> I believe it's called a poster frame, right?
It's called a 'thumbnail image'. The "poster frame" is the specific frame of a movie that has been designated to be shown in still-image representations of a movie; e.g., the poster frame is what is shown in the thumbnail image, so the concepts are related.