Could stars form outside of galaxies? Is it possible for there to be a dense enough nebula to form stars outside of any galaxy? Does a galaxy have a minimum size to produce stars? Or could you have a few dozen stars clustered together by themselves?
Yes, stars can form outside galaxies if the conditions are right. An impressive example is D100, a galaxy that is moving through a cluster so fast that the ram pressure from the ambient gas forces galactic gas out of it leaving a long tail. That tail is dense and cold enough to allow star formation, and there are newly formed clusters in it. 



In principle star formation can happen anywhere where there is dense, cold gas that can collapse gravitationally. Stars can sometimes help by forcing gas together by radiation pressure. There are intergalactic gas clouds, but they are usually too hot.  Dark matter halos are not great at capturing hot gas, but intra-cluster gas is losing energy from Bremsstrahlung so over time some of it would accumulate there. 

Space is large, so no doubt there is some star formation happening in corners where gas clouds have cooled off remotely from galaxies. After all, that was how stars and galaxies once started.