What size casserole do I use for stuffing containing 12 oz bread cubes, 8 eggs, 2 c each onion and celery, herbs, spices, etc.y

To add on to hard's answer, you can really pack that stuff down into a casserole to make it fit, then it's just a tighter consistency when done. Maybe even slicable.

It should fit into a 2.5 quart casserole, assuming you're cooking the onion and celery down a bit so they'll take up less space.