The height of the front edge to match up to my forearm, so I don't have to lean over soo much, as on a low sink.
Large enough for the biggest trays I can think of using.
Temperature controlled faucet, for mixing chemicals and setting up water baths.
The longer the sink is the more faucets I want, to avoid having to use a long hose and run back and forth to the faucet (an accident waiting to happen).
Stainless steel would be ideal, but fiberglass or varnished plywood is more realistic for most of us.
Setup with the print washer so the print washer can drain into the sink. Or if the washer is tall, set the washer at a convenient height and have a separate drain for it.
Separate part of the sink for developing film vs paper. The layout of stuff is different. This gets more important if you do sheet film (4x5 and larger).