Working with SMEs can be challenging since the people are generally tightly knit; however, it is often easier to observe effects in such a close culture. It part it will depend on exactly what aspects you are studying. The alternative would be to look at a departmental level within a large company, since these also tend to function in a similar manner. 

I agree withJosephThornton,itdepends on what your research questions are, your purpose for studying workplace learning. SMEs can have very strong learning cultures, including established career paths for their employees. I think the question is not about the size of the company but about why you want to investogate workpalce leanring and what the end outcome will be. SMEs do not have the luxry of time and layers of management that MNCs have, so are more liely to ask, what is in it for me? In repsonse tot heri contribution of their time, they should expect soemthing in return...

I would actually try to incorporate both in my study as the comany size probably influences the environment. Including both groups could show interesting diffferences related to company size.

If you take one or the other, you won't be able to prove the influence of firm size in workplace culture.  Comparing the two (SMEs and large firms) would be less economic but more comprehensive.