I recommend having TWO versions of your synopsis – a “long synopsis” and a “short synopsis.”
In past years, there used to be a fairly universal system regarding synopses. For every 35 or so pages of text you had, you would have one page of synopsis explanation. So if your book was 245 pages, double-spaced, your synopsis would be seven pages approximately. This was fairly standard, and allowed writers a decent amount of space to explain their story. I recommend doing this first. This will be your “long synopsis.”
The problem is: Sometime in the past few years, agents started to get really busy and they want to hear your story now now now. They started asking for synopses of no more than two pages.
Many agents today request specifically just that – two pages max. Some may even say one page, but two pages is generally acceptable. You have to draft a new, more concise synopsis – the “short synopsis.”
So which one do you submit? Good question. If you think your short synopsis (1-2 pages) is tight and effective, always use that. However, if you think the long synopsis is much more effective, then you will sometimes submit one and sometimes submit the other. If an agent requests two pages max, send the short one (because, naturally, you’ve been instructed to). If they just say “Send a synopsis,” and you feel your longer synopsis is far superior, and your long synopsis isn’t more than eight pages, I say just submit the long one.