Compared to the rest of the Fairfield market, the Cos Cob region 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 leases most likley are leasing quickly. You'll want to be prepared to tour spaces as quick as possible. 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 940 sqft. Most properties in this area are around 3 floors. Most buildings in this market were built in 1967 and are Class A.