You want to keep the 12 point, but there are ways to fudge and still stay in the rules. Fonts are different "sizes" even when they're all 12 point, so you might try seeing if another serif font would get you a few more lines. Garamond is narrower than Times Roman for example (and I think it reads easier myself).
You can also play with the leading (the space between lines). It's okay to single space in a synopsis if it's only a few pages. I liked to use the 1.5 spacing, as it made it easier to read, but still kept things contained to a smaller page count.
You can also delete any spaces between paragraphs, and run the text same as you do in your manuscript, with half-inch indents.
What matters most is ease of readability, and content. If it's all scrunched together to get it to fit, and it's hard on the eyes, then it won't matter how many pages you have. But if it's easy to read and the synopsis is interesting and well written, then no one will mind the extra half page. (unless they say "no more than three pages"). Print it out and see how it reads to test it.