better like your partners, be ok with the lease (location and rent and term), look at the books (any debt, how do they fund operations at the start of the year), what is the buy-in, are the key players getting older (what happens if any get hit by a bus), etc.
I prefer small firms to bigger firms, having done both. Less stress, less work and you can often make more money (i.e., non-equity at big law compared to equity at regional law).