Talk to other similar (successful) businesses in your area and find out how much they carry. Talk to your attorney and ask what they recommend. Check with your local county/city/state and find out what is required as a minimum.
Between those three you should be able to home in on a good estimate.
If you're talking GL, most are 1m/2m (occurrence, aggregate). If you're leasing office space, they'll likely have their own requirements.
I just went through this exercise and had a really good experience with http://techinsurance.com.