Compared to the rest of the Bay Area market, the San Anselmo submarket has more expensive rates, and you'll have to negotiate above asking rent to be in this hot submarket.
In the last 90 days, 2 listings have been posted online, which means listings most likley are leasing quickly. You'll want to be prepared to make an offer immediately. There aren't many listings below 2,000 square feet available, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 1,200 sqft. Most buildings in this market were built in 2004 and are Class B.