The length seems to vary depending on the agent or publisher. Russell Galen says he likes to see about 1500 words. Others want longer or shorter. Here's a How To article on writing the 1-pager, and then expanding it.

Beth Anderson's Barebones Synopsis

What's not clear to me is whether the page-limit requests are for courier or TNR, double spaced or single. The word count total for a 1-page synopsis single-spaced TNR 12 is a whole lot higher than 1-page double-spaced Courier 12. One book I have on queries and synopses states that anything that fits on a single page should be single-spaced.