Comfort: I think a great deal of determining which SEO to select is based on how comfortable the individual making the decision is with the person or company as a whole. The SEO may say all the right things, but if the prospect isn't comfortable with the relationship that will be reason enough to go with someone else. For the SEO its important to adapt their "persona" to the person they are dealing with. I have some clients that I can joke around with whenever we talk and with others I must take a more serious approach. I find what makes them most comfortable and go with it.

Size: Yes, the size of the SEO firm matters! Different people are looking for different sizes based on their needs and desires. One will be more comfortable with a single contractor working on their account for the more personalized attention, another feels that only a large firm can handle their needs. Still others are looking for something in between where they feel they'll get the personalized service yet ensure that everything isn't farmed out to sub-contractors in India. There is nothing one can do to be the perfect size other than to provide the assurances necessary that the client will get just the right amount of attention they'll need to ensure success.