I hate writing a synopsis almost as much as a query letter. 8)
I usually write a two-page synopsis when I've polished a manuscript and want to begin the querying process. I also have the two paragraph synopsis in my query letter. When I run across someone wanting a one-page synopsis, I take the two paragraphs from the query letter and expand it -- I find it easier to expand that synopsis than to cut stuff from the longer one. When I cut stuff it needs more editing to smooth it out.
In the US, many agents want to see query letter + a few sample pages of the ms. first. Which makes more sense because considering how many manuscripts can be ruled out after reading the first few paragraphs.