glass ballustrade

Building regs specify the force that ballustrades have to withstand. People have to be able to fall against them without breaking through. Translating that to the thickness of a piece of glass sounds like a rather specialist skill, and I imagine things like the types (and numbers/positions where relevant) of glass fixings (which you haven't mentioned) would be a significant factor. It would at least need to be toughened, and this case might demand laminated.

In other words, I think this is something you're going to have to find an architectural/commercial glass supplier to answer.