It would seem that your question answers itself ! Are you asking how much larger the table is relative to the X-Y work dimensions? That depends on your design and constraints. My machine has large borders, so I lost ~4" each side. Some folks have clever gantry designs to minimize this (for example the BuildYourCNC machines allow the spindle to travel over the X rails to minimize wasted Y space).
Most folks keep the Z travel to less that 6" for stability, which fits well with your stated work sizes. Don't forget that you must be able to raise the spindle high enough to change the bit, and perhaps add a dust collection shoe.