There is no exact science on this. You have to treat each hip individually and ride it a few time to get the feel dialed in. Every night they groom the hill, the pitch will likely change so you will also need to redial things in the next day. Your best bet is to "eyeball" things and use your best guess while going a little smaller. Once you have the feel of it down, you can go bigger and you will sort of "instinctively" know what is right. Like a lot of freestyle, this is more of intuitive thing than an analytical one. Just start small then ramp things up and do this every day you ride the feature.
Hips are easy to progress on because there's usually not far to go to get to the landing (unlike some kickers). The only thing to watch out for depending on the hip is not too take too much of an angle off the side of the lip itself. As said, start off small. Don't go off the lip at a 45 degree angle if you're hitting it with speed or you'll just land flat. When you hit one fast, you want to go straighter up the lip. If you hit it at an angle like 45 degrees or so, you're going to be traveling away from the landing while you're in the air.
I've seen some kids here hitting a small-ish hip and not landing because they are not rotating their board and body into the same slope direction as the landing itself too, I don't mean the angle of the landing itself, I mean the direction the landing slope is running from the top of it. You can't aim to land the same direction you took off, chances are you'll either come out on your heels or your toes. Even though you're moving with forward momentum, you still have to get that board rotated to land stable.
If this is the problem, think of it like this. If you just stood at the top of the hips landing like you're about to drop in, which direction would your board be facing if you rode down it?