This is entirely dependent on the ferrets. Every ferret has different preferences.
They do seem to prefer to be able to get their entire bodies into the box and have room to turn around, so a large box is a must IMO. I use cat-sized litter boxes, and I just use scissors to cut a dip into one side so they can get in and out easily.
Because they are cheap, most people prefer pelleted litters. You can buy wood stove pellets at many hardware/tractor supply/home improvement stores (Home Depot, Lowe’s, etc), $5 for a 40lb bag. If you use pelleted litters, you will need to completely dump the box every couple of days and refill it.
Personally, though, my ferrets won’t use pelleted litters. They stay wet a lot longer and if their feet get wet, they will hop out of the box and poop in front of it. I usually use a walnut shell litter. It’s more expensive but they are reliable with the box which is all I care about. A plus to the walnut shell litter is that it’s scoopable and I don’t have to completely dump the box 2x a week.
Some ferrets can’t use scoopable litters. if your ferret likes to dig in it, you will want to avoid them as they can get clumps stuck up their nose. Some people get around this by mixing scoopable litters with clumping litters (a layer of scoopable litter on the bottom with pellets on top).