Just in case you’ve never sent anything off to to an agent, publisher or been asked to describe your work in a few sentences, you might never have come across the writers terror that is:
Breathe, breathe, it’s allright, it’ll be fine, you just need to breathe.
A synopsis is a (very) condensed version of the whole, so no sub plots, no detailed setting descriptions, no character names beyond the main two or three. (How George RR Martin coped with this, I’ll never know…) The synopsis can be a useful thing, really, it can. It’s a short, short version of your magnum opus. Some agents/publishers will ask for a short synopsis, others for a 2-3 page one, so it’s good to know what to put in, and what to leave out.
Typically a very short synopsis will be around 400-500 words in length, and a 2-3 page synopsis will be… well, 2-3 pages. See, not that difficult?
Most people write the synopsis at the end, condensing the work down, but you can write the other way, creating a short synopsis first and using a technique commonly known as ‘snowflaking’ or the ‘snowflake method‘ to expand it out. This has the benefit of you being able to keep sections of the work in proportion, without losing sight of where it’s going. If you get new ideas along the way, you can confidently add bits in, knowing that they’ll not unbalance the whole.