Compared to the rest of the Bay Area market, the San Anselmo submarket has lower rates, so you should have no trouble finding a great deal in this depreciated submarket.
In the last 90 days, 6 listings have been posted online, which means new availabilities are probably leasing quickly. You'll want to be prepared to make an offer immediately. There aren't many listings below 2,000 square feet posted, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 1,400 sqft. Most buildings in this region were built in 1962 and are Class B.