I always find that my best reviews come when I take notes as I read, then later skim the notes for the perfect thing to say. If you already have a pool of thoughts surrounding the book from which to draw inspiration for the review, then hopefully you won't be babbling on, or searching desperately for something to say. Keep the reviews somewhat short, but don't finish until you think you've gotten the most important pieces of the book across to your reader.
I don't ramble too much when it comes to the synopsis, I leave that up to wherever I copy and paste from. I try to get right to the point but if it's a book I am especially passionate about, I tend to go on and on!
It's up to the individual reviewer. As long as they get their point across, cover the bases and fairly pinpoint what they liked or didn't like about the book, any size is fine. I never thought of the size of a review before. Most I read aren't really that long. I do think some reviewers go off the topic of the book in their reviews. In that case, yes, they should be shortened if the reviewer has this habit. I think any review that is thorough can be any length. If it's interesting it doesn't matter. But I really don't think anyone would write a review that's pages long anyway, LOL.
I like it when the reviewer mentions the "small" plot points when reviewing a book, especially if it's mine because it lets me know they really read it. Some reviewers' reviews are like they didn't even read the book and just went on what they read from a description. It is so easy to tell when a reviewer didn't read a book or just skimmed.