I think the scene is the basic unit of the plot, more or less. Which is to say that the plot is moved along with each scene that passes, and depending on how it moves the plot, and how much, that scene may need to be longer or shorter or whatever. Two characters are having a conversation that will lead into an important decision? That's moving the plot along quite far, and I'd argue that, if the decision is that important, you're probably going to want to make the scene longer, to give yourself a chance to explore the characters' route towards that decision. Someone travelling from a place where the last important thing happened to the place where the next important thing happens? If you need to show that journey for whatever reason, it will more often than not be better to keep it brief; there's much less that needs to be said.
That said, sometimes less is more. Strong emotion often tends to be unconvincing if you spend ages trying to describe it minutely; leaving things unsaid, in very short scenes that hint at the depth of emotion rather than trying to master it, sometimes works better. I'm basically restating what everyone else has said, but the thing is, scenes are as important as they need to be. Some will probably end up feeling more important than others, but figuring out which are important and which aren't is something that you do retrospectively, after you've finished writing and can look back on the story with some distance. I wouldn't worry about not knowing what's important or not as you write; go with your gut, ask yourself what matters, and that will usually do for the first draft, at least -- after which point you can, as I say, look back on it a bit more clearly.