You are going to CD 2 nb's?? OMG you are brave! And awesome I failed at first but it got easier. I would guess a regular size garbage can with a pail liner and a lid.
We use a tall plastic kitchen trash can with a flip-top lid from Target. I wash every other day, and it is only about half-full when I take the liner bag out, so I think it would fit diapers for two kids. But when they are nbs, they will use more diapers, so you may need a temporary solution until then. One thing we have done in the past is have two changing stations - one on the main level, and one in our bedroom. We used a large hanging wetbag (I got my Planetwise one from https://www.clothdiaperclearance.com - they sell seconds there) in one location, and the kitchen pail in the other location.
I just use a step trash can with a liner. I wash every 2-2 1/2 days and it's about 1/2 way full. My LO is still a newborn. I think that ought to work fine for you.
Re: Daiper Pail?