Which preposition should I use in "I work _____ a small company." at? in? for?

In the sentence "I work _____ a small company," you can use either at or for. 

"At a small company" is the more common way to say this, but both prepositions are absolutely fine and have almost exactly the same meaning. 

I think that "for a small company" puts slightly more emphasis on the fact that the speaker is an employee of a larger organization and not in control of it. But the difference is slight, and I'm not sure that all native speakers would agree with me. 

Thanks for your question!