Either 500 or 1000 ml is good, with 500 mls you will have to do more batches but it might be a bit less messy or more comfortable. (As a rule of thumb in chemistry, scaling up can make the process a bit less controlled)... I would go with the 1000 ml though, you can get a lot more work done and I don't think the scaling up concerns apply that much yet.
IIRC, I have 3 types one of which is 1000 ml or even bigger but the tap is utter shit. The other 2 are 500 ml, one of which I highly prefer over the other. In the past I have wished for a bigger and proper one and I have not at all quickly made my way through a kg using the 500 ml funnel.