On average, listings lease in the $40's per square foot per year. The San Anselmo region is experiencing a a rise in rates. In the last 3 months, the median rate rose $42 per sqft per year.
Compared to the rest of the Bay Area market, the San Anselmo region has lower rates, so you should have no trouble finding a great deal in this depreciated region.
In the last 90 days, 6 listings have come on the market, which means spaces are probably leasing quickly. You'll want to be prepared to tour spaces as quick as possible. About 50% of the listings in the San Anselmo are subleases, so you might be able to snag a great spot on a short term. 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 990 sqft. Most properties in this area are close to 2 floors. Most buildings in this area were built in 1956 and are Class B.