For 2, I would go 2X4 and at least 2 tall. You can add levels as well to give more space. It takes 32 girds to do the cage with a roof, but does not include levels. This would be 2-3 boxes of grids depending on the number in the box and what exactly what you want to build. A taller cage will need more grids.
I only go 2 grids deep so I can reach the back of the cage easily (I do have one that is 3 deep but the bottom slides out for cleaning, that is the only way it would fit in the space). I like big doors, at least 2 grids wide and preferably 2 tall. Try to not block the door opening with levels.
When you make the bottom, I find it easier to have the coroplast all on the inside of the bars. You may need something to keep them from chewing. Sides at least 4" tall are also good, but you can go taller if you want.
When I clean, I first take out all the stuff in the cage (litter box, toys, dishes, etc) and the rabbit if needed. I then shake out the blanket (fleece) and sweep up all the hay, poop and whatever else is there. Then I spray with a vinegar and water mix and wipe up with paper towel. I clean the litter boxes and bowls, put a clean blanket in and put everything back. It usually takes me about 15-20 minutes per cage to totally clean it depending on the rabbit.