Write and submit your synopsis as you do your challenge entries. There is a place to post your synopsis on the submission page. Single space with double spaces between paragraphs. The file of your chapter needs to be uploaded.
As for the length, I think I read in another thread that you have no idea where the book is going, which makes it difficult to write a synopsis at all. Of course, this doesn't mean your first chapter won't WOW the judges but I would probably take some time to at least get some idea in your mind about where you want the book to go and end up. I wouldn't say it's too short without seeing it but I just wonder how much info. you can give in only 155 words.
Then again, maybe it's plenty for the judges!
Sorry I can't be of more help. If it feels right to you, don't sweat too much over it. It's the chapter that the judges will be ultimately judging.
I say, great job in just biting the bullet and going for it. If nothing else, you've stepped up to the challenge, which already makes you a winner.