https://angel.co/ displays the number of employees that a company has right next to the job listing and generally listings there favor smaller organizations. Otherwise there's the option of manually sifting through results and cross references sources like linkedin to check employee counts. Per the HN Who's hiring thread, there were at least 26 instances of the phrase "small team" this month, but that doesn't necessarily imply the whole company is small, though I guess it's an ok filter to start with.