This is the best way I've found and we've done it for years.
Line your baking dish with aluminum foil making sure to tightly seal any seams. Have at least an inch hanging over each edge.
Spray with cooking spray and make your casserole.
Place in the freezer and freeze over night or until solid.
Take out of the freezer and lift out the casserole using the foil as a handle.
Take a second piece of foil to cover the top and seal completely. Label. Wash and return your dish to the cupboard and your wrapped casserole to the freezer.
When you want to have that dinner, find the right baking dish, spray with cooking spray and peel the foil from your casserole - thanks to the cooking spray it releases easily. Pop your perfectly-sized meal into the dish and cook/thaw as directed.
This saves you from buying a ton of baking dishes and stacks very nicely in your freezer.
You can reuse the piece of foil from the bottom of your casserole to cover your meal during baking - it's usually very flat and is still good to use.
Hope you liked this tip :-)