I think that there is no defined limit to the possible scale, just that the larger the scale, the more careful you need to be to avoid "hot spots" when it comes to temperature, as well as base or acid concentration gradients. Add all reagents dropwise, and with excellent stirring.
Naturally, with a sensitive synthesis like this one, you always should use "superclean" reagents, as any garbage put into a reaction always multiplies to give even more garbage out.