Whether you are submitting to an agent or a publisher, it is imperative that you follow their specific guidelines and requirements for submission. Each will have different requirements but often, these will include: a synopsis of your manuscript, a sample extract from the novel and a covering letter.
You can find the precise details of the relevant submission requirements on the agent's or publisher's website. Make sure you read and follow these carefully. The agent or publisher will not amend their demands just for you. Also make sure the agent or publisher you have targeted is accepting submissions at the time you want to contact them. Check that they are interested in receiving submissions from authors in your genre.
Agents are often specific about the genres they do not represent but sometimes do not specify their preferred genres. However, in larger agencies, each agent will probably specialise in particular genres and may also accept direct submissions. This will usually be detailed on the agency's website. You need to think about every part of your submission package.
Not every agent or publisher requires a synopsis as part of the early submission, but it is often requested at this point. Most will certainly ask for a synopsis at some point during the submission process so make sure you have one prepared. Indeed, it is helpful to have more than one prepared as the length of the required synopsis can vary widely.
Some agents and publishers unhelpfully require a synopsis but do not indicate their preferred length. Traditionally, agents and publishers will accept quite a long synopsis that might be up to 4000 words. In the absence of detailed instructions, you should probably ensure that your submission package includes a long synopsis of this type along with a 'blurb'. This is a much more condensed description of the story of no more than one or two paragraphs such as would ultimately be printed on the back of the published book to entice readers to buy the novel.
Of course, the publishing world has changed a great deal in the last twenty years. Agents and publishers are increasingly busy, and the accessibility of on-line submissions has increased the number of manuscripts that are sent to them. Many agents and publishers struggle to work through the slush piles of unsolicited manuscripts they receive. They are able only to give a limited time to considering whether a submission is worth taking further.
If an agent or publisher does ask for a short synopsis but without stating how long it should be, they are likely to be looking for a piece of about 1-2 pages and definitely not more than 1000 words. If they require a synopsis but do not indicate a preferred length at all, it is safer to send a short synopsis in the first instance. If they are really interested in your manuscript, they will ask to see the full manuscript and probably a detailed synopsis later.
You should already have prepared a first draft of your long synopsis for your purposes. Now you need to shape that into a synopsis that is appropriate to include in your submission package in either long or short forms. In truth, you are most likely to submit only the short synopsis initially. Nevertheless, it is worth having the longer version prepared and ready to send if requested. In the event that your book is accepted by an agent or publisher, you will no doubt find the various lengths of your synopsis are useful. Perhaps an agent might want to send a longer version of the synopsis to potential publishers, or you might find it helpful when you are briefing cover designers or even for marketing purposes.
Now consider how you can make the synopsis suitable for submission. Be aware that a short synopsis is basically a sales pitch. It is a longer version of the blurb that will appear on the back of your book. It is a document intended to engage the interest of the agent or publisher you send it to. The longer synopsis is likely to be read only when that interest has been secured, so it can be a more factual version of the story.
The synopsis must set out the story your book tells in the order in which it occurs. Therefore, if you have used flashbacks, place these where they occur in the story and if the tale is told over different time frames, make sure that these are clear in the narrative of the synopsis. Show the story holds together and makes sense. It is important that the agent or publisher can see that it is well structured and that the pace is good. Show, also, that your story has a satisfactory conclusion and give the agent or publisher confidence that you have sustained the quality of your opening chapters throughout the whole book.
Many authors find that writing the synopsis is the most difficult part of the submission to prepare. Take care and good luck with your synopsis.