Maybe under 200nm, according to the last part of the attached literature.

Article Nanoparticles: Molecular targets and cell signalling

I recomend you to read this paper:

http://www.ncbi.nlm.nih.gov/pubmed/18654486

I`ve done a huge research about that for my PhD and taking into acount the literature and my personal experience on the field i would say that the optimun nanoparticle size range would be around 20-50nm not only for celular uptake but also for an optimal biodistribution in vivo. I this context, think Its important the size to be higher than 6nm to avoid renal filtration and smaller maybe than 100-200nm to avoid acumulation in liver or spleen.