Can reducing the reservoir size for catching the drain from a shunt in the eye for glaucoma help cause back pressure?

Yes: The most common shunts are the ahmed and baerveldt implants. They come in different sizes which are designed to fit larger or smaller eyes and to a lesser extent, regulate the outflow pressures. The pressure is more dependent however on the status of the overlying conjunctival filter.

Not sure: I don't think i understand the question, or what you mean by back pressure. If you are asking if pressures can increase by making the reservoir size smaller then previous reservoir size, prob yes, but normally this would not be done.